Investment Securities - Amortized Cost, Held to Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities
Amortized Cost	$ 2,030,194	$ 1,842,691
Gross Unrealized Gains	14,292	25,419
Gross Unrealized Losses	(55,704)	(30,490)
Fair Value	1,988,782	1,837,620
U.S. Treasury securities
Schedule of Held-to-maturity Securities
Amortized Cost	138,598	138,676
Gross Unrealized Gains	3,007	6,581
Gross Unrealized Losses	0	0
Fair Value	141,605	145,257
U.S. government agency securities
Schedule of Held-to-maturity Securities
Amortized Cost	9,447	9,859
Gross Unrealized Gains	21	122
Gross Unrealized Losses	(188)	0
Fair Value	9,280	9,981
Obligations of states and state agencies
Schedule of Held-to-maturity Securities
Amortized Cost	355,219	244,272
Gross Unrealized Gains	4,940	7,083
Gross Unrealized Losses	(5,862)	(1,653)
Fair Value	354,297	249,702
Municipal bonds
Schedule of Held-to-maturity Securities
Amortized Cost	260,432	221,606
Gross Unrealized Gains	3,634	6,199
Gross Unrealized Losses	(1,577)	(28)
Fair Value	262,489	227,777
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities
Amortized Cost	615,651	465,878
Gross Unrealized Gains	8,574	13,282
Gross Unrealized Losses	(7,439)	(1,681)
Fair Value	616,786	477,479
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities
Amortized Cost	1,185,165	1,131,945
Gross Unrealized Gains	2,493	4,842
Gross Unrealized Losses	(41,208)	(18,743)
Fair Value	1,146,450	1,118,044
Trust preferred securities
Schedule of Held-to-maturity Securities
Amortized Cost	49,833	49,824
Gross Unrealized Gains	78	60
Gross Unrealized Losses	(6,577)	(9,796)
Fair Value	43,334	40,088
Corporate and other debt securities
Schedule of Held-to-maturity Securities
Amortized Cost	31,500	46,509
Gross Unrealized Gains	119	532
Gross Unrealized Losses	(292)	(270)
Fair Value	$ 31,327	$ 46,771