Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 545	$ 425
Due after one through five years	9,984	5,040
Due after five through ten years	79,118	74,921
Due after ten years	468,612	430,497
Total debt securities with a single maturity date	558,259	510,883
Total Investment Securities	1,111,446	986,565
Fair Value
Due in one year or less	548	425
Due after one through five years	10,051	5,204
Due after five through ten years	80,457	78,806
Due after ten years	465,320	442,289
Total debt securities with a single maturity date	556,376	526,724
Total Investment Securities	1,096,837	999,947
Amortized Cost
Due in one year or less	24,829	12,015
Due after one through five years	59,713	76,146
Due after five through ten years	53,004	54,441
Due after ten years	111,156	116,610
Total debt securities with a single maturity date	248,702	259,212
Total Investment Securities	522,846	560,655
Fair Value
Due in one year or less	25,203	12,158
Due after one through five years	59,075	76,334
Due after five through ten years	53,453	55,679
Due after ten years	111,881	120,657
Total debt securities with a single maturity date	249,612	264,828
Investment securities held to maturity	519,643	568,208
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
Without single maturity date	553,187	473,325
Fair Value
Without single maturity date	540,461	470,866
Amortized Cost
Without single maturity date	274,144	301,443
Fair Value
Without single maturity date	$ 270,031	303,380
Equity securities
Amortized Cost
Without single maturity date		2,357
Fair Value
Without single maturity date		2,357
Amortized Cost
Without single maturity date
Fair Value
Without single maturity date