Investment Securities (Securities with Unrealized Losses and Length of Impairment Period) (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Fair Value	$ 9,278,066	$ 7,294,303
Unrealized Losses	23,288	36,177
U.S. Government and Federal Agency Obligations [Member]
Fair Value	360,102	455,537
Government-Sponsored Enterprise Obligations [Member]
Fair Value	264,550	201,895
Unrealized Losses	81	530
Government-Sponsored Enterprise Obligations [Member] | Less Than 12 Months [Member]
Fair Value	24,894	10,850
Unrealized Losses	81	530
Government-Sponsored Enterprise Obligations [Member] | Total [Member]
Fair Value	24,894	10,850
Unrealized Losses	81	530
State and Municipal Obligations [Member]
Fair Value	1,259,384	1,119,485
Unrealized Losses	11,918	16,643
State and Municipal Obligations [Member] | Less Than 12 Months [Member]
Fair Value	109,967	345,775
Unrealized Losses	1,082	7,470
State and Municipal Obligations [Member] | 12 Months or Longer [Member]
Fair Value	111,627	82,269
Unrealized Losses	10,836	9,173
State and Municipal Obligations [Member] | Total [Member]
Fair Value	221,594	428,044
Unrealized Losses	11,918	16,643
Agency Mortgage-Backed Securities [Member]
Fair Value	4,295,194	2,491,199
Unrealized Losses	315	3,440
Agency Mortgage-Backed Securities [Member] | Less Than 12 Months [Member]
Fair Value	37,510	660,326
Unrealized Losses	315	3,440
Agency Mortgage-Backed Securities [Member] | Total [Member]
Fair Value	37,510	660,326
Unrealized Losses	315	3,440
Non-Agency Mortgage-Backed Securities [Member]
Fair Value	353,229	455,790
Unrealized Losses	8,603	14,513
Non-Agency Mortgage-Backed Securities [Member] | Less Than 12 Months [Member]
Fair Value	33,453	15,893
Unrealized Losses	989	36
Non-Agency Mortgage-Backed Securities [Member] | 12 Months or Longer [Member]
Fair Value	128,453	170,545
Unrealized Losses	7,614	14,477
Non-Agency Mortgage-Backed Securities [Member] | Total [Member]
Fair Value	161,906	186,438
Unrealized Losses	8,603	14,513
Other Asset-Backed Securities [Member]
Fair Value	2,537,251	2,354,260
Unrealized Losses	2,371	1,051
Other Asset-Backed Securities [Member] | Less Than 12 Months [Member]
Fair Value	740,357	487,822
Unrealized Losses	1,934	1,029
Other Asset-Backed Securities [Member] | 12 Months or Longer [Member]
Fair Value	77,207	24,928
Unrealized Losses	437	22
Other Asset-Backed Securities [Member] | Total [Member]
Fair Value	817,564	512,750
Unrealized Losses	2,371	1,051
Other Debt Securities [Member]
Fair Value	167,633	176,964
Equity Securities [Member]
Fair Value	40,723	39,173
Total Mortgage and Asset-Backed Securities [Member]
Fair Value	7,185,674	5,301,249
Unrealized Losses	11,289	19,004
Total Mortgage and Asset-Backed Securities [Member] | Less Than 12 Months [Member]
Fair Value	811,320	1,164,041
Unrealized Losses	3,238	4,505
Total Mortgage and Asset-Backed Securities [Member] | 12 Months or Longer [Member]
Fair Value	205,660	195,473
Unrealized Losses	8,051	14,499
Total Mortgage and Asset-Backed Securities [Member] | Total [Member]
Fair Value	1,016,980	1,359,514
Unrealized Losses	11,289	19,004
Total Temporarily Impaired Securities [Member] | Less Than 12 Months [Member]
Fair Value	946,181	1,520,666
Unrealized Losses	4,401	12,505
Total Temporarily Impaired Securities [Member] | 12 Months or Longer [Member]
Fair Value	317,287	277,742
Unrealized Losses	18,887	23,672
Total Temporarily Impaired Securities [Member] | Total [Member]
Fair Value	1,263,468	1,798,408
Unrealized Losses	$ 23,288	$ 36,177