Investment Securities - Amortized Cost, Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,133,427	$ 2,336,095
Gross Unrealized Gains	69,787	34,556
Gross Unrealized Losses	(8,338)	(11,931)
Fair Value	2,194,876	2,358,720
U.S. Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	138,268	138,352
Gross Unrealized Gains	9,147	5,761
Gross Unrealized Losses	0	0
Fair Value	147,415	144,113
U.S. government agency securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,657	7,345
Gross Unrealized Gains	264	58
Gross Unrealized Losses	0	(41)
Fair Value	6,921	7,362
Obligations of states and state agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	280,187	297,454
Gross Unrealized Gains	8,569	7,745
Gross Unrealized Losses	(545)	(529)
Fair Value	288,211	304,670
Municipal bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	186,649	203,251
Gross Unrealized Gains	6,876	5,696
Gross Unrealized Losses	0	(10)
Fair Value	193,525	208,937
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	466,836	500,705
Gross Unrealized Gains	15,445	13,441
Gross Unrealized Losses	(545)	(539)
Fair Value	481,736	513,607
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,451,581	1,620,119
Gross Unrealized Gains	44,327	14,803
Gross Unrealized Losses	(309)	(5,350)
Fair Value	1,495,599	1,629,572
Trust preferred securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37,335	37,324
Gross Unrealized Gains	48	39
Gross Unrealized Losses	(7,483)	(5,981)
Fair Value	29,900	31,382
Corporate and other debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	32,750	32,250
Gross Unrealized Gains	556	454
Gross Unrealized Losses	(1)	(20)
Fair Value	$ 33,305	$ 32,684