Investment Securities - Gross Unrealized Losses and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 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	$ 577,752	$ 1,297,600
Less than 12 months, Unrealized Losses	3,642	7,842
12 months or more, Fair Value	0	200,534
12 months or more, Unrealized Losses	0	2,517
Total, Fair Value	577,752	1,498,134
Total, Unrealized Losses	3,642	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	58,504	226,177
Less than 12 months, Unrealized Losses	156	678
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	58,504	226,177
Total, Unrealized Losses	156	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	206,197	60,639
Less than 12 months, Unrealized Losses	2,913	687
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	206,197	60,639
Total, Unrealized Losses	2,913	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	312,613	1,010,354
Less than 12 months, Unrealized Losses	558	6,456
12 months or more, Fair Value	0	200,534
12 months or more, Unrealized Losses	0	2,517
Total, Fair Value	312,613	1,210,888
Total, Unrealized Losses	558	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	292,534	667,511
Less than 12 months, Unrealized Losses	523	3,586
12 months or more, Fair Value	0	200,534
12 months or more, Unrealized Losses	0	2,517
Total, Fair Value	292,534	868,045
Total, Unrealized Losses	523	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	20,079	140,403
Less than 12 months, Unrealized Losses	35	616
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	20,079	140,403
Total, Unrealized Losses	35	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	0	176,880
Less than 12 months, Unrealized Losses	0	2,049
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	0	176,880
Total, Unrealized Losses	0	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	438	430
Less than 12 months, Unrealized Losses	15	21
12 months or more, Fair Value	0	0
12 months or more, Unrealized Losses	0	0
Total, Fair Value	438	430
Total, Unrealized Losses	$ 15	$ 21

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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