Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014 securities	Dec. 31, 2013 securities
Available-for-Sale:
Available-for-Sale, less than 12 months fair value	$ 355,102	$ 1,092,584
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,589)	(27,628)
Available-for-Sale, 12 months or more fair value	460,753	226,396
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(7,407)	(9,769)
Available-for-Sale Securities, Fair Value	815,855	1,318,980
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(8,996)	(37,397)
Held-to-Maturity:
Held-to-maturity, less than 12 months fair value	83,935	44,844
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,846)	(1,327)
Held-to-maturity, 12 months or more fair value	20,097	14,296
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(373)	(730)
Held-to-maturity Securities, Fair Value	104,032	59,140
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2,219)	(2,057)
Available-for-Sale and Held-to-Maturity:
Investment securities in an unrealized loss position (number of securities)	154	229
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, less than 12 months fair value	135,888	458,385
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(702)	(10,355)
Available-for-Sale, 12 months or more fair value	309,283	59,362
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(4,668)	(1,894)
Available-for-Sale Securities, Fair Value	445,171	517,747
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(5,370)	(12,249)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, less than 12 months fair value	219,214	634,199
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(887)	(17,273)
Available-for-Sale, 12 months or more fair value	151,380	166,930
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2,737)	(7,874)
Available-for-Sale Securities, Fair Value	370,594	801,129
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(3,624)	(25,147)
Held-to-Maturity:
Held-to-maturity, less than 12 months fair value	61,201
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(1,758)
Held-to-maturity, 12 months or more fair value	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0
Held-to-maturity Securities, Fair Value	61,201
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(1,758)
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, less than 12 months fair value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	0	0
Available-for-Sale, 12 months or more fair value	90	104
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(2)	(1)
Available-for-Sale Securities, Fair Value	90	104
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	(2)	(1)
State, county and municipal securities
Held-to-Maturity:
Held-to-maturity, less than 12 months fair value	7,979	37,550
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(13)	(1,319)
Held-to-maturity, 12 months or more fair value	20,097	14,296
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	(373)	(730)
Held-to-maturity Securities, Fair Value	28,076	51,846
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	(386)	(2,049)
Corporate securities
Held-to-Maturity:
Held-to-maturity, less than 12 months fair value	14,755	7,294
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	(75)	(8)
Held-to-maturity, 12 months or more fair value	0	0
Held-to-maturity Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Held-to-maturity Securities, Fair Value	14,755	7,294
Held-to-maturity Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ (75)	$ (8)