Investment Securities - Gross Unrealized Losses and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value	$ 742,548	$ 1,297,600
Less than 12 months, Unrealized Losses	4,289	7,842
12 months or more, Fair Value	0	200,534
12 months or more, Unrealized Losses	0	2,517
Total, Fair Value	742,548	1,498,134
Total, Unrealized Losses	4,289	10,359
Government and government-sponsored enterprises
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value	139,940	226,177
Less than 12 months, Unrealized Losses	813	678
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	139,940	226,177
Total, Unrealized Losses	813	678
Obligations of states and political subdivisions
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value	78,241	60,639
Less than 12 months, Unrealized Losses	1,087	687
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	78,241	60,639
Total, Unrealized Losses	1,087	687
Total mortgage-backed debt securities
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value	524,367	1,010,354
Less than 12 months, Unrealized Losses	2,389	6,456
12 months or more, Fair Value	0	200,534
12 months or more, Unrealized Losses	0	2,517
Total, Fair Value	524,367	1,210,888
Total, Unrealized Losses	2,389	8,973
Residential agency
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value	426,171	667,511
Less than 12 months, Unrealized Losses	1,388	3,586
12 months or more, Fair Value	0	200,534
12 months or more, Unrealized Losses	0	2,517
Total, Fair Value	426,171	868,045
Total, Unrealized Losses	1,388	6,103
Residential non-agency
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value	1,529	140,403
Less than 12 months, Unrealized Losses	2	616
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	1,529	140,403
Total, Unrealized Losses	2	616
Commercial agency
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value	96,667	176,880
Less than 12 months, Unrealized Losses	999	2,049
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	96,667	176,880
Total, Unrealized Losses	999	2,049
Commercial non-agency
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value	0	25,560
Less than 12 months, Unrealized Losses	0	205
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	0	25,560
Total, Unrealized Losses	$ 0	205
Corporate debt and trust preferred securities
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value		430
Less than 12 months, Unrealized Losses		21
12 months or more, Fair Value		0
12 months or more, Unrealized Losses		0
Total, Fair Value		430
Total, Unrealized Losses		$ 21