Investment Securities Gross Unrealized Losses and Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014 securities	Dec. 31, 2013 securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 131,789	$ 1,092,584
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(637)	(27,628)
Available-for-Sale, 12 Months or More Fair Value	585,731	226,396
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(9,813)	(9,769)
Available-for-Sale, Total Fair Value	717,520	1,318,980
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(10,450)	(37,397)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	240,262	44,844
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(2,208)	(1,327)
Held-to-Maturity, 12 Months or More Fair Value	20,586	14,296
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(454)	(730)
Held-to-Maturity, Total Fair Value	260,848	59,140
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(2,662)	(2,057)
Available-for-Sale and Held-to-Maturity:
Investment Securities in an Unrealized Loss Position (Number of Securities)	174	229
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	29,552	458,385
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(209)	(10,355)
Available-for-Sale, 12 Months or More Fair Value	388,327	59,362
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(5,631)	(1,894)
Available-for-Sale, Total Fair Value	417,879	517,747
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(5,840)	(12,249)
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	102,237	634,199
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(428)	(17,273)
Available-for-Sale, 12 Months or More Fair Value	197,313	166,930
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(4,181)	(7,874)
Available-for-Sale, Total Fair Value	299,550	801,129
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(4,609)	(25,147)
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	91	104
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	(1)	(1)
Available-for-Sale, Total Fair Value	91	104
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(1)	(1)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	240,262
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(2,208)
Held-to-Maturity, 12 Months or More Fair Value	0
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	0
Held-to-Maturity, Total Fair Value	240,262
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(2,208)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	0	37,550
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	0	(1,319)
Held-to-Maturity, 12 Months or More Fair Value	20,586	14,296
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(454)	(730)
Held-to-Maturity, Total Fair Value	20,586	51,846
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(454)	(2,049)
Corporate Debt Securities [Member]
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value		7,294
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1		(8)
Held-to-Maturity, 12 Months or More Fair Value		0
Held-to-Maturity, 12 Months or More Gross Unrealized Losses		0
Held-to-Maturity, Total Fair Value		7,294
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1		$ (8)