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, 2018	Dec. 31, 2017
Amortized Cost
Due in one year or less	$ 24,098	$ 425
Due after one through five years	17,294	5,040
Due after five through ten years	75,416	74,921
Due after ten years	513,243	430,497
Total debt securities with a single maturity date	630,051	510,883
Total Investment Securities	1,177,274	986,565
Fair Value
Due in one year or less	24,360	425
Due after one through five years	17,511	5,204
Due after five through ten years	75,498	78,806
Due after ten years	501,188	442,289
Total debt securities with a single maturity date	618,557	526,724
Total Investment Securities	1,149,162	999,947
Amortized Cost
Due in one year or less	5,161	12,015
Due after one through five years	48,155	76,146
Due after five through ten years	57,950	54,441
Due after ten years	104,476	116,610
Total debt securities with a single maturity date	215,742	259,212
Total Investment Securities	476,089	560,655
Fair Value
Due in one year or less	5,178	12,158
Due after one through five years	47,042	76,334
Due after five through ten years	58,212	55,679
Due after ten years	103,887	120,657
Total debt securities with a single maturity date	214,319	264,828
Investment securities held to maturity	468,452	568,208
U.S. Government-sponsored mortgage-backed securities
Amortized Cost
U.S. Government-sponsored mortgage-backed securities	547,223	473,325
Fair Value
Without single maturity date	530,605	470,866
Amortized Cost
Without single maturity date	260,347	301,443
Fair Value
Without single maturity date	$ 254,133	303,380
Equity securities
Amortized Cost
U.S. Government-sponsored mortgage-backed securities		2,357
Fair Value
Without single maturity date		2,357
Amortized Cost
Without single maturity date
Fair Value
Without single maturity date