Investment Securities - Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturity [Abstract]
Due in one year or less - Amortized Cost	$ 3,836
Due after one through five years - Amortized Cost	8,980
Due after five through ten years - Amortized Cost	50,220
Due after ten years - Amortized Cost	186,794
Total debt securities with a single maturity date - Amortized Cost	249,830
Total Investment Securities - Amortized Cost	517,276
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity [Abstract]
Due in one year or less - Fair Value	3,869
Due after one through five years - Fair Value	9,245
Due after five through ten years - Fair Value	52,853
Due after ten years - Fair Value	195,755
Total debt securities with a single maturity date - Fair Value	261,722
Total Investment Securities - Fair Value	538,055
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Due in one year or less - Amortized Cost	8,072
Due after one through five years - Amortized Cost	19,561
Due after five through ten years - Amortized Cost	86,271
Due after ten years - Amortized Cost	111,321
Total debt securities with a single maturity date - Amortized Cost	225,225
Total Investment Securities - Amortized Cost	651,418	631,088
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturity [Abstract]
Due in one year or less - Fair Value	8,121
Due after one through five years - Fair Value	20,155
Due after five through ten years - Fair Value	88,851
Due after ten years - Fair Value	114,451
Total debt securities with a single maturity date - Fair Value	231,578
Investment securities held to maturity - fair value	670,611	647,723
U.S. Government-sponsored mortgage-backed securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturity [Abstract]
Without single maturity date - Amortized Cost	265,740
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity [Abstract]
Without single maturity date - Fair Value	274,627
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis [Abstract]
Without single maturity date - Amortized Cost	426,193
Total Investment Securities - Amortized Cost	426,193	426,645
Held-to-maturity Securities, Debt Maturities, Single Maturity Date, Fair Value, Rolling Maturity [Abstract]
Without single maturity date - Fair Value	439,033
Investment securities held to maturity - fair value	439,033	437,660
Equity securities
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Amortized Cost Basis, Rolling Maturity [Abstract]
Without single maturity date - Amortized Cost	1,706
Available-for-sale Securities, Debt Maturities, Single Maturity Date, Rolling Maturity [Abstract]
Without single maturity date - Fair Value	$ 1,706