Investment Securities, Available-for-Sale (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Total investment securities, weighted yield	2.66%	2.64%
Total investment securities, amortized cost	$ 3,490,679	$ 3,063,032
Total investment securities, gross unrealized gains	102,692	73,018
Total investment securities, gross unrealized losses	(7,016)	(9,307)
Total investment securities, fair value	3,586,355	3,126,743
U.S. government and federal agency [Member]
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Maturing after one year through five years, weighted yield	1.62%	1.62%
Maturing after one year through five years, amortized cost	202	204
Maturing after one year through five years, gross unrealized gains	2	4
Maturing after one year through five years, fair value	204	208
U.S. government sponsored enterprises [Member]
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Maturing within one year, weighted yield		1.58%
Maturing after one year through five years, weighted yield	2.32%	2.36%
Maturing after five years through ten years, weighted yield	1.90%	1.90%
Total investment securities, weighted yield	2.32%	2.26%
Maturing within one year, amortized cost		3,979
Maturing after one year through five years, amortized cost	19,376	26,399
Maturing after five years through ten years, amortized cost	70	78
Total investment securities, amortized cost	19,446	30,456
Maturing within one year, gross unrealized gains		17
Maturing after one year through five years, gross unrealized gains	413	682
Maturing after five years through ten years, gross unrealized gains	1
Total investment securities, gross unrealized gains	414	699
Maturing within one year, fair value		3,996
Maturing after one year through five years, fair value	19,789	27,081
Maturing after five years through ten years, fair value	71	78
Total investment securities, fair value	19,860	31,155
State and local governments [Member]
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Maturing within one year, weighted yield	0.95%	1.31%
Maturing after one year through five years, weighted yield	2.11%	2.22%
Maturing after five years through ten years, weighted yield	2.83%	2.59%
Maturing after ten years, weighted yield	4.71%	4.84%
Total investment securities, weighted yield	4.16%	4.44%
Maturing within one year, amortized cost	45,469	4,786
Maturing after one year through five years, amortized cost	143,651	89,752
Maturing after five years through ten years, amortized cost	43,065	63,143
Maturing after ten years, amortized cost	918,714	845,657
Total investment securities, amortized cost	1,150,899	1,003,338
Maturing within one year, gross unrealized gains	8	3
Maturing after one year through five years, gross unrealized gains	4,652	2,660
Maturing after five years through ten years, gross unrealized gains	1,406	2,094
Maturing after ten years, gross unrealized gains	81,821	57,138
Total investment securities, gross unrealized gains	87,887	61,895
Maturing within one year, gross unrealized losses	(3)	(2)
Maturing after one year through five years, gross unrealized losses	(109)	(22)
Maturing after five years through ten years, gross unrealized losses	(50)	(19)
Maturing after ten years, gross unrealized losses	(746)	(535)
Total investment securities, gross unrealized losses	(908)	(578)
Maturing within one year, fair value	45,474	4,787
Maturing after one year through five years, fair value	148,194	92,390
Maturing after five years through ten years, fair value	44,421	65,218
Maturing after ten years, fair value	999,789	902,260
Total investment securities, fair value	1,237,878	1,064,655
Corporate bonds [Member]
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Maturing within one year, weighted yield	1.63%
Maturing after one year through five years, weighted yield	2.35%	2.55%
Maturing after five years through ten years, weighted yield	2.30%	2.38%
Total investment securities, weighted yield	2.28%	2.54%
Maturing within one year, amortized cost	21,420
Maturing after one year through five years, amortized cost	200,106	60,810
Maturing after five years through ten years, amortized cost	18,088	2,409
Total investment securities, amortized cost	239,614	63,219
Maturing within one year, gross unrealized gains	108
Maturing after one year through five years, gross unrealized gains	3,355	261
Maturing after five years through ten years, gross unrealized gains	437	21
Total investment securities, gross unrealized gains	3,900	282
Maturing within one year, gross unrealized losses	0
Maturing after one year through five years, gross unrealized losses	(28)	(1,264)
Maturing after five years through ten years, gross unrealized losses	(7)
Total investment securities, gross unrealized losses	(35)	(1,264)
Maturing within one year, fair value	21,528
Maturing after one year through five years, fair value	203,433	59,807
Maturing after five years through ten years, fair value	18,518	2,430
Total investment securities, fair value	243,479	62,237
Collateralized debt obligations [Member]
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Maturing after ten years, weighted yield	8.03%	8.03%
Maturing after ten years, amortized cost	2,847	5,648
Maturing after ten years, gross unrealized losses	(85)	(282)
Maturing after ten years, fair value	2,762	5,366
Residential mortgage-backed securities [Member]
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Total investment securities, weighted yield	1.87%	1.70%
Total investment securities, amortized cost	2,077,671	1,960,167
Total investment securities, gross unrealized gains	10,489	10,138
Total investment securities, gross unrealized losses	(5,988)	(7,183)
Total investment securities, fair value	$ 2,082,172	$ 1,963,122