Investment Securities Gross Unrealized Losses and Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015 securities	Dec. 31, 2014 securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 100,138	$ 355,102
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(241)	(1,589)
Available-for-Sale, 12 Months or More Fair Value	419,617	460,753
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(3,442)	(7,407)
Available-for-Sale, Total Fair Value	519,755	815,855
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(3,683)	(8,996)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	57,088	83,935
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(2,262)	(1,846)
Held-to-Maturity, 12 Months or More Fair Value	13,831	20,097
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(180)	(373)
Held-to-Maturity, Total Fair Value	70,919	104,032
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(2,442)	(2,219)
Available-for-Sale and Held-to-Maturity:
Investment Securities in an Unrealized Loss Position (Number of Securities)	100	154
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	76,190	135,888
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(185)	(702)
Available-for-Sale, 12 Months or More Fair Value	277,320	309,283
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(1,635)	(4,668)
Available-for-Sale, Total Fair Value	353,510	445,171
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(1,820)	(5,370)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	23,948	219,214
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(56)	(887)
Available-for-Sale, 12 Months or More Fair Value	142,213	151,380
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(1,805)	(2,737)
Available-for-Sale, Total Fair Value	166,161	370,594
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(1,861)	(3,624)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	45,264	61,201
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(2,235)	(1,758)
Held-to-Maturity, 12 Months or More Fair Value	0	0
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	0	0
Held-to-Maturity, Total Fair Value	45,264	61,201
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(2,235)	(1,758)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	1,778	7,979
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(10)	(13)
Held-to-Maturity, 12 Months or More Fair Value	13,831	20,097
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(180)	(373)
Held-to-Maturity, Total Fair Value	15,609	28,076
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(190)	(386)
Private mortgage-backed securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	0	0
Available-for-Sale, 12 Months or More Fair Value	84	90
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(2)	(2)
Available-for-Sale, Total Fair Value	84	90
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(2)	(2)
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	10,046	14,755
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(17)	(75)
Held-to-Maturity, 12 Months or More Fair Value	0	0
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	0	0
Held-to-Maturity, Total Fair Value	10,046	14,755
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	$ (17)	$ (75)