Securities Fair Value Option Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule of Fair Value Option Securities [Line Items]
Fair Value Option Securities, Fair Value	$ 819,531	$ 77,046	$ 222,409
Fair Value Option Securities, Net Unrealized Gain or Loss	1,671	(1,777)	(2,397)
U.S. Treasury
Schedule of Fair Value Option Securities [Line Items]
Fair Value Option Securities, Fair Value	0	0	222,409
Fair Value Option Securities, Net Unrealized Gain or Loss	0	0	(2,397)
U.S. government agency residential mortgage-backed securities
Schedule of Fair Value Option Securities [Line Items]
Fair Value Option Securities, Fair Value	819,531	77,046	0
Fair Value Option Securities, Net Unrealized Gain or Loss	$ 1,671	$ (1,777)	$ 0