Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	Dec. 31, 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	158	120
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	$ (82,370)	$ (17,680)
Less Than 12 Months Fair Value	4,086,005	2,682,621
12 Months or Longer Unrealized Losses	(18,715)	(27,262)
12 Months or Longer Fair Value	520,369	709,322
Total Unrealized Losses	(101,085)	(44,942)
Total Fair Value	4,606,374	3,391,943
U.S. Treasury securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(13,164)	(2,150)
Less Than 12 Months Fair Value	392,473	499,976
Total Unrealized Losses	(13,164)	(2,150)
Total Fair Value	392,473	499,976
Government-sponsored enterprises debt securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(7,056)	(324)
Less Than 12 Months Fair Value	207,651	70,808
Total Unrealized Losses	(7,056)	(324)
Total Fair Value	207,651	70,808
Government agency mortgage-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(4,822)	(508)
Less Than 12 Months Fair Value	185,663	55,982
Total Unrealized Losses	(4,822)	(508)
Total Fair Value	185,663	55,982
Government-sponsored enterprises mortgage-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(4,034)
Less Than 12 Months Fair Value	195,848
Total Unrealized Losses	(4,034)
Total Fair Value	195,848
Non-government asset-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(3)	(143)
Less Than 12 Months Fair Value	5,202	95,310
12 Months or Longer Unrealized Losses	(6)
12 Months or Longer Fair Value	7,381
Total Unrealized Losses	(9)	(143)
Total Fair Value	12,583	95,310
Collateralized mortgage obligations: Government agency
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(51,484)	(11,423)
Less Than 12 Months Fair Value	2,847,103	1,428,423
12 Months or Longer Unrealized Losses	(7,310)	(12,153)
12 Months or Longer Fair Value	233,706	354,335
Total Unrealized Losses	(58,794)	(23,576)
Total Fair Value	3,080,809	1,782,758
Collateralized mortgage obligations: Government-sponsored enterprises
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(1,807)	(3,132)
Less Than 12 Months Fair Value	252,065	532,122
12 Months or Longer Unrealized Losses	(11,399)	(15,109)
12 Months or Longer Fair Value	279,282	354,987
Total Unrealized Losses	(13,206)	(18,241)
Total Fair Value	$ 531,347	$ 887,109