Securities Available for Sale and Held to Maturity (1) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Fair value [Abstract]
Estimated fair value, less than 12 months	$ 44,859		$ 44,859		$ 51,327
Estimated fair value, 12 months or more	957		957		202
Total estimated fair value	45,816		45,816		51,529
Gross unrealized losses [Abstract]
Gross unrealized (losses), less than 12 months	(211)		(211)		(189)
Gross unrealized (losses), 12 months or more	(3)		(3)		(2)
Total gross unrealized (losses)	(214)		(214)		(191)
Gains (losses) on investments [Abstract]
Gross realized gains	(1)	33	1,041	91
Available for Sale, amortized cost [Abstract]
Due in one year or less	12,642		12,642
Due after one year through five years	74,924		74,924
Due after five years through ten years	381,607		381,607
Due after ten years	33,061		33,061
Total amortized cost, before mortgage-backed-securities	502,234		502,234
Mortgage-backed securities	2,146,552		2,146,552
Total amortized cost	2,648,786		2,648,786
Available for Sale, estimated fair value [Abstract]
Due in one year or less	12,717		12,717
Due after one year through five years	78,933		78,933
Due after five years through ten years	393,544		393,544
Due after ten years	33,517		33,517
Total fair value, before mortgage-backed-securities	518,711		518,711
Mortgage-backed securities	2,205,735		2,205,735
Securities available for sale (at fair value)	2,724,446		2,724,446		2,468,993
Held to maturity, amortized cost [Abstract]
Due in one year or less	2,002		2,002
Due after one year through five years	17,347		17,347
Due after five years through ten years	17,566		17,566
Due after ten years	754		754
Total amortized cost, before mortgage-backed-securities	37,669		37,669
Mortgage-backed securities	7,815		7,815
Securities held to maturity	45,484		45,484		57,705
Held to maturity, estimated fair value [Abstract]
Due in one year or less	2,022		2,022
Due after one year through five years	18,878		18,878
Due after five years through ten years	20,188		20,188
Due after ten years	804		804
Total fair value, before mortgage-backed-securities	41,892		41,892
Mortgage-backed securities	8,380		8,380
Securities held to maturity (at fair value)	50,272		50,272
Available-for-sale Securities [Member]
Gains (losses) on investments [Abstract]
Proceeds from calls and sales of securities	2,710	1,175	37,536	24,171
Gross realized gains	(1)	5	1,038	57
Held-to-maturity Securities [Member]
Gains (losses) on investments [Abstract]
Proceeds from calls of securities	0	2,355	175	3,645
Gross realized gains	0	28	3	34
Obligations of States and Political Subdivisions [Member]
Fair value [Abstract]
Estimated fair value, less than 12 months	1,886		1,886		3,368
Estimated fair value, 12 months or more	957		957		202
Total estimated fair value	2,843		2,843		3,570
Gross unrealized losses [Abstract]
Gross unrealized (losses), less than 12 months	(5)		(5)		(12)
Gross unrealized (losses), 12 months or more	(3)		(3)		(2)
Total gross unrealized (losses)	(8)		(8)		(14)
Other Residential Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Fair value [Abstract]
Estimated fair value, less than 12 months	19,971		19,971		1,069
Estimated fair value, 12 months or more	0		0		0
Total estimated fair value	19,971		19,971		1,069
Gross unrealized losses [Abstract]
Gross unrealized (losses), less than 12 months	(19)		(19)		(1)
Gross unrealized (losses), 12 months or more	0		0		0
Total gross unrealized (losses)	(19)		(19)		(1)
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Fair value [Abstract]
Estimated fair value, less than 12 months	16,404		16,404		46,890
Estimated fair value, 12 months or more	0		0		0
Total estimated fair value	16,404		16,404		46,890
Gross unrealized losses [Abstract]
Gross unrealized (losses), less than 12 months	(164)		(164)		(176)
Gross unrealized (losses), 12 months or more	0		0		0
Total gross unrealized (losses)	(164)		(164)		(176)
Asset-backed Securities [Member]
Fair value [Abstract]
Estimated fair value, less than 12 months	6,598		6,598
Estimated fair value, 12 months or more	0		0
Total estimated fair value	6,598		6,598
Gross unrealized losses [Abstract]
Gross unrealized (losses), less than 12 months	(23)		(23)
Gross unrealized (losses), 12 months or more	0		0
Total gross unrealized (losses)	$ (23)		$ (23)