Investment Securities Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Thousands	Jun. 30, 2015 USD ($) securities	Dec. 31, 2014 USD ($) securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 360,915	$ 355,102
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(1,940)	(1,589)
Available-for-Sale, 12 Months or More Fair Value	373,131	460,753
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(4,829)	(7,407)
Available-for-Sale, Total Fair Value	734,046	815,855
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(6,769)	(8,996)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	136,004	83,935
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(3,657)	(1,846)
Held-to-Maturity, 12 Months or More Fair Value	13,513	20,097
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(359)	(373)
Held-to-Maturity, Total Fair Value	149,517	104,032
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	$ (4,016)	$ (2,219)
Available-for-Sale and Held-to-Maturity:
Investment Securities in an Unrealized Loss Position (Number of Securities) | securities	190	154
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 106,516	$ 135,888
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(460)	(702)
Available-for-Sale, 12 Months or More Fair Value	239,339	309,283
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(1,844)	(4,668)
Available-for-Sale, Total Fair Value	345,855	445,171
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(2,304)	(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	254,399	219,214
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(1,480)	(887)
Available-for-Sale, 12 Months or More Fair Value	133,709	151,380
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(2,983)	(2,737)
Available-for-Sale, Total Fair Value	388,108	370,594
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(4,463)	(3,624)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	85,286	61,201
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(3,288)	(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	85,286	61,201
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(3,288)	(1,758)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	19,427	7,979
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(165)	(13)
Held-to-Maturity, 12 Months or More Fair Value	13,513	20,097
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(359)	(373)
Held-to-Maturity, Total Fair Value	32,940	28,076
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(524)	(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	83	90
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(2)	(2)
Available-for-Sale, Total Fair Value	83	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	31,291	14,755
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(204)	(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	31,291	14,755
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	$ (204)	$ (75)