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, 2019	Dec. 31, 2018
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value	$ 1,297,600	$ 106,329
Less than 12 months, Unrealized Losses	7,842	184
12 months or more, Fair Value	200,534	1,365,299
12 months or more, Unrealized Losses	2,517	36,023
Total, Fair Value	1,498,134	1,471,628
Total, Unrealized Losses	10,359	36,207
Gross unrealized losses and fair value of debt securities held to maturity that are in a loss position
Less than 12 months, Fair Value	7,116	3,074
Less than 12 months, Unrealized Losses	72	14
12 months or more, Fair Value	9,209	31,738
12 months or more, Unrealized Losses	105	1,031
Total, Fair Value	16,325	34,812
Total, Unrealized Losses	177	1,045
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	226,177
Less than 12 months, Unrealized Losses	678
12 months or more, Fair Value	0
12 months or more, Unrealized Losses	0
Total, Fair Value	226,177
Total, Unrealized Losses	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	60,639	3,620
Less than 12 months, Unrealized Losses	687	0
12 months or more, Fair Value	0	526,817
12 months or more, Unrealized Losses	0	9,479
Total, Fair Value	60,639	530,437
Total, Unrealized Losses	687	9,479
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	1,010,354
Less than 12 months, Unrealized Losses	6,456
12 months or more, Fair Value	200,534
12 months or more, Unrealized Losses	2,517
Total, Fair Value	1,210,888
Total, Unrealized Losses	8,973
Agency mortgage-backed 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	667,511	102,709
Less than 12 months, Unrealized Losses	3,586	184
12 months or more, Fair Value	200,534	838,482
12 months or more, Unrealized Losses	2,517	26,544
Total, Fair Value	868,045	941,191
Total, Unrealized Losses	6,103	26,728
Gross unrealized losses and fair value of debt securities held to maturity that are in a loss position
Less than 12 months, Fair Value	7,116	3,074
Less than 12 months, Unrealized Losses	72	14
12 months or more, Fair Value	9,209	31,738
12 months or more, Unrealized Losses	105	1,031
Total, Fair Value	16,325	34,812
Total, Unrealized Losses	177	$ 1,045
Non-agency mortgage-backed 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	140,403
Less than 12 months, Unrealized Losses	616
12 months or more, Fair Value	0
12 months or more, Unrealized Losses	0
Total, Fair Value	140,403
Total, Unrealized Losses	616
Agency collateralized mortgage obligations
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value	176,880
Less than 12 months, Unrealized Losses	2,049
12 months or more, Fair Value	0
12 months or more, Unrealized Losses	0
Total, Fair Value	176,880
Total, Unrealized Losses	2,049
Non-agency collateralized mortgage obligations
Gross unrealized losses and fair value of debt securities available for sale that are in a loss position
Less than 12 months, Fair Value	25,560
Less than 12 months, Unrealized Losses	205
12 months or more, Fair Value	0
12 months or more, Unrealized Losses	0
Total, Fair Value	25,560
Total, Unrealized Losses	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

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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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