Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	Dec. 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	196	158
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	$ (29,116)	$ (82,370)
Less Than 12 Months Fair Value	2,119,812	4,086,005
12 Months or Longer Unrealized Losses	(93,742)	(18,715)
12 Months or Longer Fair Value	3,052,744	520,369
Total Unrealized Losses	(122,858)	(101,085)
Total Fair Value	5,172,556	4,606,374
U.S. Treasury securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(994)	(13,164)
Less Than 12 Months Fair Value	48,182	392,473
12 Months or Longer Unrealized Losses	(11,127)
12 Months or Longer Fair Value	344,073
Total Unrealized Losses	(12,121)	(13,164)
Total Fair Value	392,255	392,473
Government-sponsored enterprises debt securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(642)	(7,056)
Less Than 12 Months Fair Value	59,358	207,651
12 Months or Longer Unrealized Losses	(6,469)
12 Months or Longer Fair Value	183,243
Total Unrealized Losses	(7,111)	(7,056)
Total Fair Value	242,601	207,651
Government agency mortgage-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(976)	(4,822)
Less Than 12 Months Fair Value	200,963	185,663
12 Months or Longer Unrealized Losses	(4,492)
12 Months or Longer Fair Value	150,427
Total Unrealized Losses	(5,468)	(4,822)
Total Fair Value	351,390	185,663
Government-sponsored enterprises mortgage-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(1)	(4,034)
Less Than 12 Months Fair Value	63	195,848
12 Months or Longer Unrealized Losses	(4,129)
12 Months or Longer Fair Value	168,342
Total Unrealized Losses	(4,130)	(4,034)
Total Fair Value	168,405	195,848
Non-government asset-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses		(3)
Less Than 12 Months Fair Value		5,202
12 Months or Longer Unrealized Losses		(6)
12 Months or Longer Fair Value		7,381
Total Unrealized Losses		(9)
Total Fair Value		12,583
Collateralized mortgage obligations: Government agency
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(23,236)	(51,484)
Less Than 12 Months Fair Value	1,473,170	2,847,103
12 Months or Longer Unrealized Losses	(53,510)	(7,310)
12 Months or Longer Fair Value	1,803,338	233,706
Total Unrealized Losses	(76,746)	(58,794)
Total Fair Value	3,276,508	3,080,809
Collateralized mortgage obligations: Government-sponsored enterprises
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(3,203)	(1,807)
Less Than 12 Months Fair Value	327,435	252,065
12 Months or Longer Unrealized Losses	(14,015)	(11,399)
12 Months or Longer Fair Value	403,321	279,282
Total Unrealized Losses	(17,218)	(13,206)
Total Fair Value	730,756	$ 531,347
States and political subdivisions
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(64)
Less Than 12 Months Fair Value	10,641
Total Unrealized Losses	(64)
Total Fair Value	$ 10,641