Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	Mar. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Securities in the available-for-sale portfolio in a continuous loss position
Number of individual securities in a continuous loss position | security	168	158
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	$ (79,933)	$ (82,370)
Less Than 12 Months Fair Value	4,325,082	4,086,005
12 Months or Longer Unrealized Losses	(19,414)	(18,715)
12 Months or Longer Fair Value	517,454	520,369
Total Unrealized Losses	(99,347)	(101,085)
Total Fair Value	4,842,536	4,606,374
U.S. Treasury securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(11,515)	(13,164)
Less Than 12 Months Fair Value	393,811	392,473
Total Unrealized Losses	(11,515)	(13,164)
Total Fair Value	393,811	392,473
Government-sponsored enterprises debt securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(6,473)	(7,056)
Less Than 12 Months Fair Value	208,235	207,651
Total Unrealized Losses	(6,473)	(7,056)
Total Fair Value	208,235	207,651
Government agency mortgage-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(5,220)	(4,822)
Less Than 12 Months Fair Value	175,536	185,663
Total Unrealized Losses	(5,220)	(4,822)
Total Fair Value	175,536	185,663
Government-sponsored enterprises mortgage-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(3,831)	(4,034)
Less Than 12 Months Fair Value	188,351	195,848
Total Unrealized Losses	(3,831)	(4,034)
Total Fair Value	188,351	195,848
Non-government asset-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(2)	(3)
Less Than 12 Months Fair Value	3,341	5,202
12 Months or Longer Unrealized Losses	(3)	(6)
12 Months or Longer Fair Value	4,693	7,381
Total Unrealized Losses	(5)	(9)
Total Fair Value	8,034	12,583
Collateralized mortgage obligations: Government agency
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(50,147)	(51,484)
Less Than 12 Months Fair Value	2,920,271	2,847,103
12 Months or Longer Unrealized Losses	(8,254)	(7,310)
12 Months or Longer Fair Value	246,336	233,706
Total Unrealized Losses	(58,401)	(58,794)
Total Fair Value	3,166,607	3,080,809
Collateralized mortgage obligations: Government-sponsored enterprises
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(2,745)	(1,807)
Less Than 12 Months Fair Value	435,537	252,065
12 Months or Longer Unrealized Losses	(11,157)	(11,399)
12 Months or Longer Fair Value	266,425	279,282
Total Unrealized Losses	(13,902)	(13,206)
Total Fair Value	$ 701,962	$ 531,347