Investments (Unrealized Losses) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Investment securities available for sale:
Less than 12 months, fair value	$ 924,252	$ 664,243	$ 2,752,137
12 months or more, fair value	10,089	9,433	1,123
Total fair value	934,341	673,676	2,753,260
Less than 12 months, unrealized losses	1,671	581	5,502
12 months or more, unrealized losses	133	127	35
Total unrealized losses	1,804	708	5,537
U. S. Treasury
Investment securities available for sale:
Less than 12 months, fair value	25,060	120,045	579,673
12 months or more, fair value	0	0	0
Total fair value	25,060	120,045	579,673
Less than 12 months, unrealized losses	1	12	305
12 months or more, unrealized losses	0	0	0
Total unrealized losses	1	12	305
Government Agency
Investment securities available for sale:
Less than 12 months, fair value	548,618	407,498	2,143,742
12 months or more, fair value	0	0	0
Total fair value	548,618	407,498	2,143,742
Less than 12 months, unrealized losses	274	337	5,040
12 months or more, unrealized losses	0	0	0
Total unrealized losses	274	337	5,040
Residential Mortgage-Backed Securities
Investment securities available for sale:
Less than 12 months, fair value	349,746	135,880	28,595
12 months or more, fair value	10,089	9,433	1,113
Total fair value	359,835	145,313	29,708
Less than 12 months, unrealized losses	1,379	214	156
12 months or more, unrealized losses	133	127	35
Total unrealized losses	1,512	341	191
Investment securities held to maturity:
Less than 12 months, fair value	0	0	0
Less than 12 months, unrealized losses	0	0	0
12 months or more, fair value	16	17	20
12 months or more, unrealized losses	27	27	27
Total fair value	16	17	20
Total unrealized losses	27	27	27
Other Debt Obligations
Investment securities available for sale:
Less than 12 months, fair value	828	820
12 months or more, fair value	0	0
Total fair value	828	820
Less than 12 months, unrealized losses	17	18
12 months or more, unrealized losses	0	0
Total unrealized losses	17	18
State, County and Municipal
Investment securities available for sale:
Less than 12 months, fair value			127
12 months or more, fair value			10
Total fair value			137
Less than 12 months, unrealized losses			1
12 months or more, unrealized losses			0
Total unrealized losses			$ 1