Regulatory Capital Requirements - Stockholder's Equity (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Regulatory Capital Requirements
Stockholder's equity	$ 2,501,008		$ 2,715,530	$ 2,501,008	$ 2,715,530	$ 2,736,941	$ 2,675,040
Decrease in stockholder's equity				$ 235,900
Decrease in stockholder's equity (as a percent)				8.60%
Net income	$ 54,860	$ 65,531	$ 54,558	$ 120,391	$ 108,680