Investment Securities - Gross Unrealized Losses and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Mar. 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	$ 672,044	$ 1,297,600
Less than 12 months, Unrealized Losses	8,044	7,842
12 months or more, Fair Value	0	200,534
12 months or more, Unrealized Losses	0	2,517
Total, Fair Value	672,044	1,498,134
Total, Unrealized Losses	8,044	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	182,694	226,177
Less than 12 months, Unrealized Losses	496	678
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	182,694	226,177
Total, Unrealized Losses	496	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	145,406	60,639
Less than 12 months, Unrealized Losses	769	687
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	145,406	60,639
Total, Unrealized Losses	769	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	343,544	1,010,354
Less than 12 months, Unrealized Losses	6,728	6,456
12 months or more, Fair Value	0	200,534
12 months or more, Unrealized Losses	0	2,517
Total, Fair Value	343,544	1,210,888
Total, Unrealized Losses	6,728	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	480	667,511
Less than 12 months, Unrealized Losses	13	3,586
12 months or more, Fair Value	0	200,534
12 months or more, Unrealized Losses	0	2,517
Total, Fair Value	480	868,045
Total, Unrealized Losses	13	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	251,453	140,403
Less than 12 months, Unrealized Losses	4,544	616
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	251,453	140,403
Total, Unrealized Losses	4,544	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	89,607	176,880
Less than 12 months, Unrealized Losses	2,156	2,049
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	89,607	176,880
Total, Unrealized Losses	2,156	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	2,004	25,560
Less than 12 months, Unrealized Losses	15	205
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	2,004	25,560
Total, Unrealized Losses	15	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	400	430
Less than 12 months, Unrealized Losses	51	21
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	400	430
Total, Unrealized Losses	$ 51	$ 21