Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investment Securities
Available-for-sale Securities, Amortized Cost Basis	$ 909,368	$ 848,316
Available-for-sale Securities, Gross Unrealized Gain	2,176	11,145
Available-for-sale Securities, Gross Unrealized Loss	(7,511)	(8,994)
Investment securities, available-for-sale	904,033	850,467
Contractual maturities of debt securities available-for-sale, Amortized Cost
Due in one year or less	94,929
Due after one year through five years	493,262
Due after five years through ten years	31,850
Due after ten years	0
Mortgage-backed securities	289,327
Total debt securities available-for-sale	909,368
Contractual maturities of debt securities available-for-sale, Fair Value
Due in one year or less	95,016
Due after one year through five years	489,615
Due after five years through ten years	31,492
Due after ten years	0
Mortgage-backed securities	287,910
Total debt securities available-for-sale	904,033
U.S. Treasury and Federal agencies securities
Investment Securities
Available-for-sale Securities, Amortized Cost Basis	471,508	424,495
Available-for-sale Securities, Gross Unrealized Gain	57	809
Available-for-sale Securities, Gross Unrealized Loss	(3,446)	(4,471)
Investment securities, available-for-sale	468,119	420,833
U.S. States and political subdivisions securities
Investment Securities
Available-for-sale Securities, Amortized Cost Basis	116,260	133,509
Available-for-sale Securities, Gross Unrealized Gain	648	1,036
Available-for-sale Securities, Gross Unrealized Loss	(908)	(1,570)
Investment securities, available-for-sale	116,000	132,975
Mortgage-backed securities - Federal agencies
Investment Securities
Available-for-sale Securities, Amortized Cost Basis	289,327	252,981
Available-for-sale Securities, Gross Unrealized Gain	1,456	2,175
Available-for-sale Securities, Gross Unrealized Loss	(2,873)	(2,582)
Investment securities, available-for-sale	287,910	252,574
Corporate debt securities
Investment Securities
Available-for-sale Securities, Amortized Cost Basis	31,573	35,266
Available-for-sale Securities, Gross Unrealized Gain	5	111
Available-for-sale Securities, Gross Unrealized Loss	(284)	(301)
Investment securities, available-for-sale	31,294	35,076
Foreign government and other securities
Investment Securities
Available-for-sale Securities, Amortized Cost Basis	700	800
Available-for-sale Securities, Gross Unrealized Gain	10	7
Available-for-sale Securities, Gross Unrealized Loss	0	0
Investment securities, available-for-sale	$ 710	807
Total debt securities
Investment Securities
Available-for-sale Securities, Amortized Cost Basis		847,051
Available-for-sale Securities, Gross Unrealized Gain		4,138
Available-for-sale Securities, Gross Unrealized Loss		(8,924)
Investment securities, available-for-sale		842,265
Marketable equity securities
Investment Securities
Available-for-sale Securities, Amortized Cost Basis		1,265
Available-for-sale Securities, Gross Unrealized Gain		7,007
Available-for-sale Securities, Gross Unrealized Loss		(70)
Investment securities, available-for-sale		$ 8,202