Investment Securities - Amortized Cost, Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,747,357	$ 1,796,404
Gross Unrealized Gains	3,941	1,995
Gross Unrealized Losses	(28,192)	(48,855)
Fair Value	1,723,106	1,749,544
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	50,969	50,975
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,115)	(1,669)
Fair Value	49,854	49,306
U.S. government agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	34,734	36,844
Gross Unrealized Gains	57	71
Gross Unrealized Losses	(238)	(638)
Fair Value	34,553	36,277
Obligations of states and state agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	97,111	100,777
Gross Unrealized Gains	189	18
Gross Unrealized Losses	(2,840)	(3,682)
Fair Value	94,460	97,113
Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	99,286	101,207
Gross Unrealized Gains	500	209
Gross Unrealized Losses	(296)	(1,437)
Fair Value	99,490	99,979
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	196,397	201,984
Gross Unrealized Gains	689	227
Gross Unrealized Losses	(3,136)	(5,119)
Fair Value	193,950	197,092
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,429,847	1,469,059
Gross Unrealized Gains	3,093	1,484
Gross Unrealized Losses	(23,324)	(40,761)
Fair Value	1,409,616	1,429,782
Corporate and other debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	35,410	37,542
Gross Unrealized Gains	102	213
Gross Unrealized Losses	(379)	(668)
Fair Value	$ 35,133	$ 37,087