Investment Securities - Amortized Cost, Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,615,073	$ 1,796,404
Gross Unrealized Gains	16,910	1,995
Gross Unrealized Losses	(3,921)	(48,855)
Fair Value	1,628,062	1,749,544
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	70,947	50,975
Gross Unrealized Gains	27	0
Gross Unrealized Losses	(9)	(1,669)
Fair Value	70,965	49,306
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	29,479	36,844
Gross Unrealized Gains	579	71
Gross Unrealized Losses	(21)	(638)
Fair Value	30,037	36,277
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	83,794	100,777
Gross Unrealized Gains	510	18
Gross Unrealized Losses	(83)	(3,682)
Fair Value	84,221	97,113
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	95,381	101,207
Gross Unrealized Gains	1,019	209
Gross Unrealized Losses	(154)	(1,437)
Fair Value	96,246	99,979
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	179,175	201,984
Gross Unrealized Gains	1,529	227
Gross Unrealized Losses	(237)	(5,119)
Fair Value	180,467	197,092
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,300,050	1,469,059
Gross Unrealized Gains	14,191	1,484
Gross Unrealized Losses	(3,654)	(40,761)
Fair Value	1,310,587	1,429,782
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	35,422	37,542
Gross Unrealized Gains	584	213
Gross Unrealized Losses	0	(668)
Fair Value	$ 36,006	$ 37,087