Investment Securities Gross Unrealized Losses and Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 securities	Dec. 31, 2012 securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 1,184,354	$ 344,180
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	27,085	1,689
Available-for-Sale, 12 Months or More Fair Value	110	137
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	3	1
Available-for-Sale, Total Fair Value	1,184,464	344,317
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	27,088	1,690
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	58,918	28,701
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(1,852)	(181)
Held-to-Maturity, 12 Months or More Fair Value	468	557
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(28)	(8)
Held-to-Maturity, Total Fair Value	59,386	29,258
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(1,880)	(189)
Available-for-Sale and Held-to-Maturity:
Investment Securities in an Unrealized Loss Position (Number of Securities)	211	69
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	472,232	93,982
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	9,862	163
Available-for-Sale, 12 Months or More Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	0	0
Available-for-Sale, Total Fair Value	472,232	93,982
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	9,862	163
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	712,122	250,198
Available-for-sale Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	17,223	1,526
Available-for-Sale, 12 Months or More Fair Value	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	0	0
Available-for-Sale, Total Fair Value	712,122	250,198
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	17,223	1,526
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	110	137
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 months or Longer, Aggregate Loss1	3	1
Available-for-Sale, Total Fair Value	110	137
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss1	3	1
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	51,606	19,389
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(1,833)	(168)
Held-to-Maturity, 12 Months or More Fair Value	468	557
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(28)	(8)
Held-to-Maturity, Total Fair Value	52,074	19,946
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	(1,861)	(176)
Corporate Debt Securities [Member]
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	7,312	9,312
Held-to-maturity Securities, Continuous Unrealized Loss Position, Less Than 12 months, Aggregate Loss1	(19)	(13)
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	7,312	9,312
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Loss1	$ (19)	$ (13)