S.1241 is a proposed bill that would require you to declare all digital currency when entering the US (I assume if you leave, you must declare upon return), or risk asset seizure. Keep an eye on.
S.1241 (see section 13)
https://www.congress.gov/bill/115th-congress/senate-bill/1241/text
From Investopedia
http://www.investopedia.com/news/congress-bill-require-declaration-digital-currencies-us-border-bitcoin-homeland-security/?lgl=rira-baseline-vertical