Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	Mar. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security
Securities in the available-for-sale portfolio in a continuous loss position
Number of individual securities in a continuous loss position | security	160	154
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	$ (790)	$ (1)
12 Months or More Unrealized Losses	(74,233)	(142,161)
Total Unrealized Losses	(75,023)	(142,162)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	262,685	172
12 Months or More Fair Value	3,523,071	3,619,406
Total Fair Value	3,785,756	3,619,578
Government agency debt securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(13)
Total Unrealized Losses	(13)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	24,765
Total Fair Value	24,765
Government-sponsored enterprises debt securities
Time in Continuous Loss, Unrealized Losses
12 Months or More Unrealized Losses	(3,123)	(6,778)
Total Unrealized Losses	(3,123)	(6,778)
Time in Continuous Loss, Fair Value
12 Months or More Fair Value	161,595	157,939
Total Fair Value	161,595	157,939
Government agency mortgage-backed securities
Time in Continuous Loss, Unrealized Losses
12 Months or More Unrealized Losses	(6,925)	(15,174)
Total Unrealized Losses	(6,925)	(15,174)
Time in Continuous Loss, Fair Value
12 Months or More Fair Value	371,035	373,891
Total Fair Value	371,035	373,891
Government-sponsored enterprises mortgage-backed securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses		(1)
12 Months or More Unrealized Losses	(4,059)	(5,293)
Total Unrealized Losses	(4,059)	(5,294)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value		172
12 Months or More Fair Value	123,430	125,869
Total Fair Value	123,430	126,041
Collateralized mortgage obligations: Government agency
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(327)
12 Months or More Unrealized Losses	(49,773)	(97,171)
Total Unrealized Losses	(50,100)	(97,171)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	99,455
12 Months or More Fair Value	2,417,727	2,475,532
Total Fair Value	2,517,182	2,475,532
Collateralized mortgage obligations: Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(450)
12 Months or More Unrealized Losses	(10,353)	(17,745)
Total Unrealized Losses	(10,803)	(17,745)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	138,465
12 Months or More Fair Value	449,284	486,175
Total Fair Value	$ 587,749	$ 486,175