Investment Securities Gross Unrealized Losses and Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013 securities	Dec. 31, 2012 securities
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	$ 548,138	$ 344,180
Available-for-Sale, Less Than 12 Months Gross Unrealized Losses	(3,843)	(1,689)
Available-for-Sale, 12 Months or More Fair Value	125	137
Available-for-Sale, 12 Months or More Gross Unrealized Losses	(1)	(1)
Available-for-Sale, Total Fair Value	548,263	344,317
Available-for-Sale, Total Gross Unrealized Losses	(3,844)	(1,690)
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	27,427	28,701
Held-to-Maturity, Less than 12 Months Gross Unrealized Losses	(282)	(181)
Held-to-Maturity, 12 Months or More Fair Value	525	557
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(6)	(8)
Held-to-Maturity, Total Fair Value	27,952	29,258
Held-to-Maturity, Total Gross Unrealized Losses	(288)	(189)
Available-for-Sale and Held-to-Maturity:
Investment Securities in an Unrealized Loss Position (Number of Securities)	91	69
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value	182,957	93,982
Available-for-Sale, Less Than 12 Months Gross Unrealized Losses	(342)	(163)
Available-for-Sale, 12 Months or More Fair Value	0	0
Available-for-Sale, 12 Months or More Gross Unrealized Losses	0	0
Available-for-Sale, Total Fair Value	182,957	93,982
Available-for-Sale, Total Gross Unrealized Losses	(342)	(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	365,181	250,198
Available-for-Sale, Less Than 12 Months Gross Unrealized Losses	(3,501)	(1,526)
Available-for-Sale, 12 Months or More Fair Value	0	0
Available-for-Sale, 12 Months or More Gross Unrealized Losses	0	0
Available-for-Sale, Total Fair Value	365,181	250,198
Available-for-Sale, Total Gross Unrealized Losses	(3,501)	(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, Less Than 12 Months Gross Unrealized Losses	0	0
Available-for-Sale, 12 Months or More Fair Value	125	137
Available-for-Sale, 12 Months or More Gross Unrealized Losses	(1)	(1)
Available-for-Sale, Total Fair Value	125	137
Available-for-Sale, Total Gross Unrealized Losses	(1)	(1)
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	24,188	19,389
Held-to-Maturity, Less than 12 Months Gross Unrealized Losses	(276)	(168)
Held-to-Maturity, 12 Months or More Fair Value	525	557
Held-to-Maturity, 12 Months or More Gross Unrealized Losses	(6)	(8)
Held-to-Maturity, Total Fair Value	24,713	19,946
Held-to-Maturity, Total Gross Unrealized Losses	(282)	(176)
Corporate Debt Securities [Member]
Held-to-Maturity:
Held-to-Maturity, Less than 12 Months Fair Value	3,239	9,312
Held-to-Maturity, Less than 12 Months Gross Unrealized Losses	(6)	(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	3,239	9,312
Held-to-Maturity, Total Gross Unrealized Losses	$ (6)	$ (13)