Securities - Securities Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,216,617		$ 2,851,479
Gross Unrealized Gains	70,144		31,205
Gross Unrealized Losses	(6,877)		(9,298)
Fair Value	3,279,884		2,873,386
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	22,193
Gross Unrealized Gains	139
Gross Unrealized Losses	0
Fair Value	22,332
Allowance for securities held-to-maturity	(220)	$ 220
Securities held-to-maturity, at amortized cost, net	22,193		21,997
Fair value, net of allowance	22,332
Equity securities	55,021		42,136
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	13,999		33,939
Gross Unrealized Gains	128		137
Gross Unrealized Losses	0		(1)
Fair Value	14,127		34,075
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	658,607		249,502
Gross Unrealized Gains	4,731		758
Gross Unrealized Losses	(4,047)		(1,836)
Fair Value	659,291		248,424
Collateralized mortgage obligations ("CMOs")
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,532,516		1,547,805
Gross Unrealized Gains	34,132		14,893
Gross Unrealized Losses	(1,665)		(5,027)
Fair Value	1,564,983		1,557,671
Other mortgage-backed securities ("MBSs")
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	624,725		678,804
Gross Unrealized Gains	16,068		7,728
Gross Unrealized Losses	(592)		(1,848)
Fair Value	640,201		684,684
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	226,149		228,632
Gross Unrealized Gains	10,694		5,898
Gross Unrealized Losses	(43)		(99)
Fair Value	236,800		234,431
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	22,413		21,997
Gross Unrealized Gains	139		0
Gross Unrealized Losses	0		(763)
Fair Value	22,552		21,234
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	160,621		112,797
Gross Unrealized Gains	4,391		1,791
Gross Unrealized Losses	(530)		(487)
Fair Value	$ 164,482		$ 114,101

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