Investment Securities - Gross Unrealized Losses and Fair Value of Debt Securities Available for Sale and Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 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	$ 940,201	$ 106,329
Less than 12 months, Unrealized Losses	3,305	184
12 months or more, Fair Value	210,896	1,365,299
12 months or more, Unrealized Losses	2,848	36,023
Total, Fair Value	1,151,097	1,471,628
Total, Unrealized Losses	6,153	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	4,031	3,074
Less than 12 months, Unrealized Losses	52	14
12 months or more, Fair Value	11,674	31,738
12 months or more, Unrealized Losses	142	1,031
Total, Fair Value	15,705	34,812
Total, Unrealized Losses	194	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	233,756
Less than 12 months, Unrealized Losses	521
12 months or more, Fair Value	0
12 months or more, Unrealized Losses	0
Total, Fair Value	233,756
Total, Unrealized Losses	521
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	273,967	3,620
Less than 12 months, Unrealized Losses	609	0
12 months or more, Fair Value	0	526,817
12 months or more, Unrealized Losses	0	9,479
Total, Fair Value	273,967	530,437
Total, Unrealized Losses	609	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	432,053
Less than 12 months, Unrealized Losses	2,150
12 months or more, Fair Value	210,896
12 months or more, Unrealized Losses	2,848
Total, Fair Value	642,949
Total, Unrealized Losses	4,998
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	241,408	102,709
Less than 12 months, Unrealized Losses	1,479	184
12 months or more, Fair Value	210,896	838,482
12 months or more, Unrealized Losses	2,848	26,544
Total, Fair Value	452,304	941,191
Total, Unrealized Losses	4,327	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	4,031	3,074
Less than 12 months, Unrealized Losses	52	14
12 months or more, Fair Value	11,674	31,738
12 months or more, Unrealized Losses	142	1,031
Total, Fair Value	15,705	34,812
Total, Unrealized Losses	194	$ 1,045
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	37,750
Less than 12 months, Unrealized Losses	101
12 months or more, Fair Value	0
12 months or more, Unrealized Losses	0
Total, Fair Value	37,750
Total, Unrealized Losses	101
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	152,895
Less than 12 months, Unrealized Losses	570
12 months or more, Fair Value	0
12 months or more, Unrealized Losses	0
Total, Fair Value	152,895
Total, Unrealized Losses	570
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	425
Less than 12 months, Unrealized Losses	25
12 months or more, Fair Value	0
12 months or more, Unrealized Losses	0
Total, Fair Value	425
Total, Unrealized Losses	$ 25