Securities - Securities Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	$ 40,690	$ 30,806
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,768,238	2,311,920
Gross Unrealized Gains	32,342	4,405
Gross Unrealized Losses	(7,264)	(44,316)
Fair Value	2,793,316	2,272,009
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23,277	10,176
Fair Value	22,405
U.S. treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	42,946	37,925
Gross Unrealized Gains	162	17
Gross Unrealized Losses	(21)	(175)
Fair Value	43,087	37,767
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	187,085	144,125
Gross Unrealized Gains	813	45
Gross Unrealized Losses	(660)	(1,607)
Fair Value	187,238	142,563
Collateralized mortgage obligations (CMOs)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,533,298	1,336,531
Gross Unrealized Gains	19,111	3,362
Gross Unrealized Losses	(2,681)	(24,684)
Fair Value	1,549,728	1,315,209
Other mortgage-backed securities (MBSs)
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	666,454	477,665
Gross Unrealized Gains	6,768	520
Gross Unrealized Losses	(2,815)	(11,251)
Fair Value	670,407	466,934
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	234,028	229,600
Gross Unrealized Gains	4,420	461
Gross Unrealized Losses	(106)	(2,874)
Fair Value	238,342	227,187
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	23,277	10,176
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(872)	(305)
Fair Value	22,405	9,871
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	104,427	86,074
Gross Unrealized Gains	1,068	0
Gross Unrealized Losses	(981)	(3,725)
Fair Value	104,514	82,349
Equity securities
Debt and Equity Securities, FV-NI [Line Items]
Equity securities	$ 40,690	$ 30,806

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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