Investment Securities Gross Unrealized Losses and Fair Values of Investment Securities (Details) $ in Thousands	Sep. 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	$ 132,180	$ 355,102
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(191)	(1,589)
Available-for-Sale, 12 Months or More Fair Value	211,552	460,753
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(1,445)	(7,407)
Available-for-Sale, Total Fair Value	343,732	815,855
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(1,636)	(8,996)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	36,274	83,935
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(3,156)	(1,846)
Held-to-Maturity, 12 Months or More Fair Value	37,837	20,097
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(758)	(373)
Held-to-Maturity, Total Fair Value	74,111	104,032
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	$ (3,914)	$ (2,219)
Available-for-Sale and Held-to-Maturity:
Investment Securities in an Unrealized Loss Position (Number of Securities) | securities	113	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	(85)	(702)
Available-for-Sale, 12 Months or More Fair Value	131,365	309,283
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(265)	(4,668)
Available-for-Sale, Total Fair Value	237,881	445,171
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(350)	(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	25,664	219,214
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(106)	(887)
Available-for-Sale, 12 Months or More Fair Value	80,116	151,380
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(1,178)	(2,737)
Available-for-Sale, Total Fair Value	105,780	370,594
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(1,284)	(3,624)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	14,834	61,201
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(3,081)	(1,758)
Held-to-Maturity, 12 Months or More Fair Value	27,344	0
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(584)	0
Held-to-Maturity, Total Fair Value	42,178	61,201
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(3,665)	(1,758)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	6,398	7,979
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(26)	(13)
Held-to-Maturity, 12 Months or More Fair Value	10,493	20,097
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(174)	(373)
Held-to-Maturity, Total Fair Value	16,891	28,076
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(200)	(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	71	90
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(2)	(2)
Available-for-Sale, Total Fair Value	71	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	15,042	14,755
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(49)	(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	15,042	14,755
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	$ (49)	$ (75)