Amortized Cost and Approximate Fair Values of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Schedule of Available for Sale and Held-to-Maturity Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains	$ 66	$ 38	$ 298	$ 56
Available-for-sale Securities, Gross Realized Losses	0	0	(3)	0
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,945,977		1,945,977		1,983,629
Available-for-Sale, Gross Unrealized Gains	34,483		34,483		33,927
Available-for-Sale, Gross Unrealized Losses	(1,306)		(1,306)		(692)
Available-for-Sale, Estimated Fair Value	1,979,154		1,979,154		2,016,864
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	187,573		187,573		152,781
Held-to-Maturity, Gross Unrealized Gains	11,552		11,552		9,113
Held-to-Maturity, Gross Unrealized Losses	(47)		(47)		(17)
Held-to-Maturity, Estimated Fair Value	199,078		199,078		161,877
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Amortized Cost	914,676		914,676		1,134,427
Available-for-Sale, Gross Unrealized Gains	4,648		4,648		4,353
Available-for-Sale, Gross Unrealized Losses	(39)		(39)		(662)
Available-for-Sale, Estimated Fair Value	919,285		919,285		1,138,118
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Amortized Cost	1,030,703		1,030,703		848,444
Available-for-Sale, Gross Unrealized Gains	29,824		29,824		29,567
Available-for-Sale, Gross Unrealized Losses	(1,266)		(1,266)		(14)
Available-for-Sale, Estimated Fair Value	1,059,261		1,059,261		877,997
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Amortized Cost	598		598		758
Available-for-Sale, Gross Unrealized Gains	11		11		7
Available-for-Sale, Gross Unrealized Losses	(1)		(1)		(16)
Available-for-Sale, Estimated Fair Value	608		608		749
State, county and municipal securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	181,792		181,792		152,619
Held-to-Maturity, Gross Unrealized Gains	11,489		11,489		9,113
Held-to-Maturity, Gross Unrealized Losses	(47)		(47)		(17)
Held-to-Maturity, Estimated Fair Value	193,234		193,234		161,715
Corporate securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	5,658		5,658
Held-to-Maturity, Gross Unrealized Gains	63		63
Held-to-Maturity, Gross Unrealized Losses	0		0
Held-to-Maturity, Estimated Fair Value	5,721		5,721
Other securities
Held-to-Maturity:
Held-to-Maturity, Amortized Cost	123		123		162
Held-to-Maturity, Gross Unrealized Gains	0		0		0
Held-to-Maturity, Gross Unrealized Losses	0		0		0
Held-to-Maturity, Estimated Fair Value	$ 123		$ 123		$ 162