Investment Securities - Unrealized Gross Losses and Fair Values of Securities in a Continuous Loss Position (Details) $ in Thousands	Sep. 30, 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	24	118
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	$ (3,349)	$ (9,658)
12 Months or More Unrealized Losses	(29)	(14,329)
Total Unrealized Losses	(3,378)	(23,987)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	468,497	1,314,806
12 Months or More Fair Value	15,575	994,155
Total Fair Value	484,072	2,308,961
U.S. Treasury and government agency debt securities
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(1)
Total Unrealized Losses	(1)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	4,976
Total Fair Value	4,976
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-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(150)	(115)
12 Months or More Unrealized Losses		(1,558)
Total Unrealized Losses	(150)	(1,673)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	51,125	76,481
12 Months or More Fair Value		109,025
Total Fair Value	51,125	185,506
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
Commercial - Government agency
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(2,844)
Total Unrealized Losses	(2,844)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	188,392
Total Fair Value	188,392
Commercial - Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(22)	(634)
Total Unrealized Losses	(22)	(634)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	70,587	38,062
Total Fair Value	70,587	38,062
Government agency
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(275)	(8,049)
12 Months or More Unrealized Losses	(27)	(8,299)
Total Unrealized Losses	(302)	(16,348)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	101,340	969,762
12 Months or More Fair Value	7,475	565,764
Total Fair Value	108,815	1,535,526
Government-sponsored enterprises
Time in Continuous Loss, Unrealized Losses
Less Than 12 Months Unrealized Losses	(57)	(583)
12 Months or More Unrealized Losses	(2)	(3,326)
Total Unrealized Losses	(59)	(3,909)
Time in Continuous Loss, Fair Value
Less Than 12 Months Fair Value	52,077	180,785
12 Months or More Fair Value	8,100	209,752
Total Fair Value	$ 60,177	$ 390,537