Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	Mar. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Securities in the available-for-sale portfolio in a continuous loss position
Number of individual securities in a continuous loss position | security	55	118
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	$ (6,628)	$ (9,658)
12 Months or More Unrealized Losses	(3,795)	(14,329)
Total Unrealized Losses	(10,423)	(23,987)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	815,348	1,314,806
12 Months or More Fair Value	253,514	994,155
Total Fair Value	1,068,862	2,308,961
Government-sponsored enterprises debt securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses		(277)
Total Unrealized Losses		(277)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value		49,716
Total Fair Value		49,716
Residential - Government agency
Time in Continuous Loss, Unrealized Losses
12 Months or More Unrealized Losses		(1,146)
Total Unrealized Losses		(1,146)
Time in Continuous Loss, Fair Value
12 Months or More Fair Value		109,614
Total Fair Value		109,614
Residential - Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(211)	(115)
12 Months or More Unrealized Losses	(93)	(1,558)
Total Unrealized Losses	(304)	(1,673)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	95,144	76,481
12 Months or More Fair Value	10,191	109,025
Total Fair Value	105,335	185,506
Commercial - Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(1,196)	(634)
Total Unrealized Losses	(1,196)	(634)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	95,097	38,062
Total Fair Value	95,097	38,062
Government agency
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(3,979)	(8,049)
12 Months or More Unrealized Losses	(1,354)	(8,299)
Total Unrealized Losses	(5,333)	(16,348)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	480,482	969,762
12 Months or More Fair Value	94,077	565,764
Total Fair Value	574,559	1,535,526
Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(1,242)	(583)
12 Months or More Unrealized Losses	(2,348)	(3,326)
Total Unrealized Losses	(3,590)	(3,909)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	144,625	180,785
12 Months or More Fair Value	149,246	209,752
Total Fair Value	$ 293,871	$ 390,537