Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 674	$ 2,703
Due after one through five years	8,970	16,359
Due after five through ten years	84,977	60,614
Due after ten years	358,326	281,234
Total debt securities with a single maturity date	452,947	360,910
Total Investment Securities	878,694	696,189
Fair Value
Due in one year or less	675	2,717
Due after one through five years	9,398	17,068
Due after five through ten years	88,832	62,241
Due after ten years	365,784	281,763
Total debt securities with a single maturity date	464,689	363,789
Total Investment Securities	890,721	696,862
Amortized Cost
Due in one year or less	7,329	2,046
Due after one through five years	79,466	61,921
Due after five through ten years	53,423	61,606
Due after ten years	120,657	121,857
Total debt securities with a single maturity date	260,875	247,430
Total Investment Securities	578,166	607,643
Fair Value
Due in one year or less	7,417	2,047
Due after one through five years	80,423	63,193
Due after five through ten years	54,573	61,145
Due after ten years	123,511	121,906
Total debt securities with a single maturity date	265,924	248,291
Investment securities held to maturity	587,270	611,933
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
Without single maturity date	423,390	313,459
Fair Value
Without single maturity date	423,675	312,292
Amortized Cost
Without single maturity date	317,291	360,213
Total Investment Securities	317,291	360,213
Fair Value
Without single maturity date	321,346	363,642
Investment securities held to maturity	321,346	363,642
Equity securities
Amortized Cost
Without single maturity date	2,357	21,820
Fair Value
Without single maturity date	2,357	20,781
Amortized Cost
Without single maturity date
Fair Value
Without single maturity date