Investments (Unrealized Losses) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011
Investment securities available for sale:
Less than 12 months, fair value	$ 384,897	$ 1,547,490	$ 1,076,407
12 months or more, fair value	5,782	1,390	2,100
Total fair value	390,679	1,548,880	1,078,507
Less than 12 months, unrealized losses	392	1,846	2,583
12 months or more, unrealized losses	49	43	50
Total unrealized losses	441	1,889	2,633
U. S. Treasury
Investment securities available for sale:
Less than 12 months, fair value	195,444	151,269
12 months or more, fair value	0	0
Total fair value	195,444	151,269
Less than 12 months, unrealized losses	30	30
12 months or more, unrealized losses	0	0
Total unrealized losses	30	30
Government Agency
Investment securities available for sale:
Less than 12 months, fair value	135,927	1,336,763	1,051,017
12 months or more, fair value	502	0	0
Total fair value	136,429	1,336,763	1,051,017
Less than 12 months, unrealized losses	78	1,512	2,435
12 months or more, unrealized losses	0	0	0
Total unrealized losses	78	1,512	2,435
Residential Mortgage-Backed Securities
Investment securities available for sale:
Less than 12 months, fair value	53,526	59,458	25,390
12 months or more, fair value	5,280	1,380	1,675
Total fair value	58,806	60,838	27,065
Less than 12 months, unrealized losses	284	304	148
12 months or more, unrealized losses	49	42	50
Total unrealized losses	333	346	198
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	18	21	22
12 months or more, unrealized losses	27	26	26
Total fair value	18	21	22
Total unrealized losses	27	26	26
State, County and Municipal
Investment securities available for sale:
Less than 12 months, fair value		0	0
12 months or more, fair value		10	425
Total fair value		10	425
Less than 12 months, unrealized losses		0	0
12 months or more, unrealized losses		1	0
Total unrealized losses		$ 1	$ 0