Amortized Cost and Fair Value of Investment Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Total investment securities, weighted yield	3.06%	2.71%
Total investment securities, amortized cost	$ 3,300,610	$ 3,587,677
Total investment securities, gross unrealized gains	52,064	101,778
Total investment securities, gross unrealized losses	(33,721)	(6,450)
Total investment securities, fair value	3,318,953	3,683,005
U.S. government and federal agency
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Maturing within one year, weighted yield		1.62%
Maturing within one year, amortized cost		201
Maturing within one year, gross unrealized gains		1
Maturing within one year, gross unrealized losses		0
Maturing within one year, fair value		202
U.S. government sponsored enterprises
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Maturing after one year through five years, weighted yield	2.29%	2.30%
Maturing after five years through ten years, weighted yield	1.82%	2.03%
Total investment securities, weighted yield	2.29%	2.29%
Maturing after one year through five years, amortized cost	11,278	17,064
Maturing after five years through ten years, amortized cost	38	44
Total investment securities, amortized cost	11,316	17,108
Maturing after one year through five years, gross unrealized gains	240	371
Maturing after five years through ten years, gross unrealized gains	0	1
Total investment securities, gross unrealized gains	240	372
Maturing after one year through five years, gross unrealized losses	0	0
Maturing after five years through ten years, gross unrealized losses	0	0
Total investment securities, gross unrealized losses	0	0
Maturing after one year through five years, fair value	11,518	17,435
Maturing after five years through ten years, fair value	38	45
Total investment securities, fair value	11,556	17,480
State and local governments
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Maturing within one year, weighted yield	2.18%	2.01%
Maturing after one year through five years, weighted yield	2.10%	2.11%
Maturing after five years through ten years, weighted yield	3.17%	2.95%
Maturing after ten years, weighted yield	4.46%	4.70%
Total investment securities, weighted yield	4.10%	4.29%
Maturing within one year, amortized cost	6,059	4,288
Maturing after one year through five years, amortized cost	162,569	149,497
Maturing after five years through ten years, amortized cost	57,892	38,346
Maturing after ten years, amortized cost	1,104,890	935,897
Total investment securities, amortized cost	1,331,410	1,128,028
Maturing within one year, gross unrealized gains	52	28
Maturing after one year through five years, gross unrealized gains	3,096	4,142
Maturing after five years through ten years, gross unrealized gains	1,122	1,102
Maturing after ten years, gross unrealized gains	28,571	82,823
Total investment securities, gross unrealized gains	32,841	88,095
Maturing within one year, gross unrealized losses	0	(2)
Maturing after one year through five years, gross unrealized losses	(431)	(142)
Maturing after five years through ten years, gross unrealized losses	(832)	(99)
Maturing after ten years, gross unrealized losses	(21,604)	(1,362)
Total investment securities, gross unrealized losses	(22,867)	(1,605)
Maturing within one year, fair value	6,111	4,314
Maturing after one year through five years, fair value	165,234	153,497
Maturing after five years through ten years, fair value	58,182	39,349
Maturing after ten years, fair value	1,111,857	1,017,358
Total investment securities, fair value	1,341,384	1,214,518
Corporate bonds
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Maturing within one year, weighted yield	2.10%	1.73%
Maturing after one year through five years, weighted yield	2.10%	2.22%
Maturing after five years through ten years, weighted yield	2.22%	2.23%
Total investment securities, weighted yield	2.10%	2.19%
Maturing within one year, amortized cost	73,064	18,412
Maturing after one year through five years, amortized cost	358,034	250,027
Maturing after five years through ten years, amortized cost	11,058	16,144
Total investment securities, amortized cost	442,156	284,583
Maturing within one year, gross unrealized gains	736	51
Maturing after one year through five years, gross unrealized gains	2,890	4,018
Maturing after five years through ten years, gross unrealized gains	0	381
Total investment securities, gross unrealized gains	3,626	4,450
Maturing within one year, gross unrealized losses	0	0
Maturing after one year through five years, gross unrealized losses	(2,852)	(238)
Maturing after five years through ten years, gross unrealized losses	(85)	0
Total investment securities, gross unrealized losses	(2,937)	(238)
Maturing within one year, fair value	73,800	18,463
Maturing after one year through five years, fair value	358,072	253,807
Maturing after five years through ten years, fair value	10,973	16,525
Total investment securities, fair value	442,845	288,795
Collateralized debt obligations
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Maturing after ten years, weighted yield		8.03%
Maturing after ten years, amortized cost		1,708
Maturing after ten years, gross unrealized gains		0
Maturing after ten years, gross unrealized losses		0
Maturing after ten years, fair value		1,708
Residential mortgage-backed securities
Comparison of amortized cost and estimated fair value of investment securities, available-for-sale
Total investment securities, weighted yield	2.44%	1.95%
Total investment securities, amortized cost	1,515,728	2,156,049
Total investment securities, gross unrealized gains	15,357	8,860
Total investment securities, gross unrealized losses	(7,917)	(4,607)
Total investment securities, fair value	$ 1,523,168	$ 2,160,302