Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	Sep. 30, 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	169	158
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	$ (19,305)	$ (82,370)
Less Than 12 Months Fair Value	2,266,486	4,086,005
12 Months or Longer Unrealized Losses	(49,709)	(18,715)
12 Months or Longer Fair Value	2,165,651	520,369
Total Unrealized Losses	(69,014)	(101,085)
Total Fair Value	4,432,137	4,606,374
U.S. Treasury securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(802)	(13,164)
Less Than 12 Months Fair Value	97,031	392,473
12 Months or Longer Unrealized Losses	(7,125)
12 Months or Longer Fair Value	299,735
Total Unrealized Losses	(7,927)	(13,164)
Total Fair Value	396,766	392,473
Government-sponsored enterprises debt securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(1,471)	(7,056)
Less Than 12 Months Fair Value	111,554	207,651
12 Months or Longer Unrealized Losses	(3,128)
12 Months or Longer Fair Value	98,558
Total Unrealized Losses	(4,599)	(7,056)
Total Fair Value	210,112	207,651
Government agency mortgage-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(2,080)	(4,822)
Less Than 12 Months Fair Value	111,991	185,663
12 Months or Longer Unrealized Losses	(1,422)
12 Months or Longer Fair Value	47,796
Total Unrealized Losses	(3,502)	(4,822)
Total Fair Value	159,787	185,663
Government-sponsored enterprises mortgage-backed securities
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(441)	(4,034)
Less Than 12 Months Fair Value	36,211	195,848
12 Months or Longer Unrealized Losses	(2,611)
12 Months or Longer Fair Value	141,003
Total Unrealized Losses	(3,052)	(4,034)
Total Fair Value	177,214	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	(12,237)	(51,484)
Less Than 12 Months Fair Value	1,589,897	2,847,103
12 Months or Longer Unrealized Losses	(26,642)	(7,310)
12 Months or Longer Fair Value	1,336,841	233,706
Total Unrealized Losses	(38,879)	(58,794)
Total Fair Value	2,926,738	3,080,809
Collateralized mortgage obligations: Government-sponsored enterprises
Time in Continuous Loss
Less Than 12 Months Unrealized Losses	(2,274)	(1,807)
Less Than 12 Months Fair Value	319,802	252,065
12 Months or Longer Unrealized Losses	(8,781)	(11,399)
12 Months or Longer Fair Value	241,718	279,282
Total Unrealized Losses	(11,055)	(13,206)
Total Fair Value	$ 561,520	$ 531,347