Investment Securities - Amortized Cost, Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 2,317,033	$ 2,336,095
Gross Unrealized Gains	73,789	34,556
Gross Unrealized Losses	(7,884)	(11,931)
Fair Value	2,382,938	2,358,720
U.S. Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	138,311	138,352
Gross Unrealized Gains	9,813	5,761
Gross Unrealized Losses	0	0
Fair Value	148,124	144,113
U.S. government agency securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	6,794	7,345
Gross Unrealized Gains	126	58
Gross Unrealized Losses	0	(41)
Fair Value	6,920	7,362
Obligations of states and state agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	289,701	297,454
Gross Unrealized Gains	7,189	7,745
Gross Unrealized Losses	(577)	(529)
Fair Value	296,313	304,670
Municipal bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	190,997	203,251
Gross Unrealized Gains	4,914	5,696
Gross Unrealized Losses	(3)	(10)
Fair Value	195,908	208,937
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	480,698	500,705
Gross Unrealized Gains	12,103	13,441
Gross Unrealized Losses	(580)	(539)
Fair Value	492,221	513,607
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,616,150	1,620,119
Gross Unrealized Gains	51,333	14,803
Gross Unrealized Losses	(80)	(5,350)
Fair Value	1,667,403	1,629,572
Trust preferred securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37,330	37,324
Gross Unrealized Gains	26	39
Gross Unrealized Losses	(7,223)	(5,981)
Fair Value	30,133	31,382
Corporate and other debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37,750	32,250
Gross Unrealized Gains	388	454
Gross Unrealized Losses	(1)	(20)
Fair Value	$ 38,137	$ 32,684