Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	Jun. 30, 2016 USD ($) security	Dec. 31, 2015 USD ($) security
Securities in the available-for-sale portfolio in a continuous loss position
Number of individual securities in a continuous loss position | security	38	120
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	$ (491)	$ (17,680)
Less Than 12 Months Fair Value	206,268	2,682,621
12 Months or Longer Unrealized Losses	(8,051)	(27,262)
12 Months or Longer Fair Value	584,127	709,322
Total Unrealized Losses	(8,542)	(44,942)
Total Fair Value	790,395	3,391,943
U.S. Treasury securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses		(2,150)
Less Than 12 Months Fair Value		499,976
Total Unrealized Losses		(2,150)
Total Fair Value		499,976
Non-government securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses		(324)
Less Than 12 Months Fair Value		70,808
Total Unrealized Losses		(324)
Total Fair Value		70,808
Government agency mortgage-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses		(508)
Less Than 12 Months Fair Value		55,982
Total Unrealized Losses		(508)
Total Fair Value		55,982
Non-government asset-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(13)	(143)
Less Than 12 Months Fair Value	30,996	95,310
12 Months or Longer Unrealized Losses	(3)
12 Months or Longer Fair Value	5,093
Total Unrealized Losses	(16)	(143)
Total Fair Value	36,089	95,310
Collateralized mortgage obligations: Government agency
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(460)	(11,423)
Less Than 12 Months Fair Value	137,915	1,428,423
12 Months or Longer Unrealized Losses	(2,698)	(12,153)
12 Months or Longer Fair Value	246,933	354,335
Total Unrealized Losses	(3,158)	(23,576)
Total Fair Value	384,848	1,782,758
Collateralized mortgage obligations: Government-sponsored enterprises
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(18)	(3,132)
Less Than 12 Months Fair Value	37,357	532,122
12 Months or Longer Unrealized Losses	(5,350)	(15,109)
12 Months or Longer Fair Value	332,101	354,987
Total Unrealized Losses	(5,368)	(18,241)
Total Fair Value	$ 369,458	$ 887,109