Securities Available for Sale and Held to Maturity (1) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Fair value [Abstract]
Estimated Fair Value, Less than 12 Months	$ 1,767,368		$ 1,767,368		$ 25,345
Estimated Fair Value, 12 Months or More	569		569		16,339
Estimated Fair Value, Total	1,767,937		1,767,937		41,684
Gross unrealized losses [Abstract]
Gross Unrealized (Losses), Less than 12 Months	(43,104)		(43,104)		(57)
Gross Unrealized (Losses), 12 Months or More	(3)		(3)		(154)
Gross Unrealized (Losses), Total	(43,107)		(43,107)		(211)
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairments			0	0
Gains (losses) on investments [Abstract]
Gross realized gains (losses)	174	(8)	378	1,042
Securities Available for Sale, Amortized Cost [Abstract]
Due in one year or less	8,863		8,863
Due after one year through five years	115,644		115,644
Due after five years through ten years	369,444		369,444
Due after ten years	217,918		217,918
Total amortized cost, before Mortgage-backed securities	711,869		711,869
Mortgage-backed securities	2,797,277		2,797,277
Total, Securities Available for Sale, Amortized Cost	3,509,146		3,509,146
Securities Available for Sale, Estimated Fair Value [Abstract]
Due in one year or less	8,910		8,910
Due after one year through five years	118,874		118,874
Due after five years through ten years	370,670		370,670
Due after ten years	221,077		221,077
Total, Estimated Fair Value, before Mortgage-backed securities	719,531		719,531
Mortgage-backed securities	2,792,152		2,792,152
Total, Securities Available for Sale, Estimated Fair Value	3,511,683		3,511,683		2,657,745
Securities Held to Maturity, Amortized Cost [Abstract]
Due in one year or less	1,136		1,136
Due after one year through five years	14,142		14,142
Due after five years through ten years	14,262		14,262
Due after ten years	755		755
Total, Amortized Cost, before Mortgage-backed securities	30,295		30,295
Mortgage-backed securities	40,043		40,043
Total, Securities Held to Maturity, Amortized Cost	70,338		70,338		42,188
Securities Held to Maturity, Estimated Fair Value [Abstract]
Due in one year or less	1,154		1,154
Due after one year through five years	15,288		15,288
Due after five years through ten years	16,368		16,368
Due after ten years	794		794
Total, Estimated Fair Value, before Mortgage-backed securities	33,604		33,604
Mortgage-backed securities	38,329		38,329
Total, Securities Held to Maturity, Estimated Fair Value	71,933		71,933		46,888
Available-for-sale Securities [Member]
Gains (losses) on investments [Abstract]
Proceeds from calls and sales of securities	29,031	34,826	64,778	34,826
Gross realized gains (losses)	174	(11)	378	1,039
Held-to-maturity Securities [Member]
Gains (losses) on investments [Abstract]
Proceeds from calls and sales of securities	0	175	0	175
Gross realized gains (losses)	0	3	0	3
U.S. Government Agency Obligations Issued by U.S. Government Agencies [Member]
Fair value [Abstract]
Estimated Fair Value, Less than 12 Months	25,207		25,207
Estimated Fair Value, 12 Months or More	0		0
Estimated Fair Value, Total	25,207		25,207
Gross unrealized losses [Abstract]
Gross Unrealized (Losses), Less than 12 Months	(739)		(739)
Gross Unrealized (Losses), 12 Months or More	0		0
Gross Unrealized (Losses), Total	(739)		(739)
Securities Held to Maturity, Amortized Cost [Abstract]
Total, Securities Held to Maturity, Amortized Cost	0		0		0
Securities Held to Maturity, Estimated Fair Value [Abstract]
Total, Securities Held to Maturity, Estimated Fair Value	0		0		0
U.S. Government Agency Obligations Issued by U.S. Government Sponsored Agencies [Member]
Fair value [Abstract]
Estimated Fair Value, Less than 12 Months	133,503		133,503
Estimated Fair Value, 12 Months or More	0		0
Estimated Fair Value, Total	133,503		133,503
Gross unrealized losses [Abstract]
Gross Unrealized (Losses), Less than 12 Months	(5,150)		(5,150)
Gross Unrealized (Losses), 12 Months or More	0		0
Gross Unrealized (Losses), Total	(5,150)		(5,150)
Securities Held to Maturity, Amortized Cost [Abstract]
Total, Securities Held to Maturity, Amortized Cost	0		0		0
Securities Held to Maturity, Estimated Fair Value [Abstract]
Total, Securities Held to Maturity, Estimated Fair Value	0		0		0
Obligations of States and Political Subdivisions [Member]
Fair value [Abstract]
Estimated Fair Value, Less than 12 Months	29,391		29,391		5,878
Estimated Fair Value, 12 Months or More	569		569		0
Estimated Fair Value, Total	29,960		29,960		5,878
Gross unrealized losses [Abstract]
Gross Unrealized (Losses), Less than 12 Months	(718)		(718)		(16)
Gross Unrealized (Losses), 12 Months or More	(3)		(3)		0
Gross Unrealized (Losses), Total	(721)		(721)		(16)
Securities Held to Maturity, Amortized Cost [Abstract]
Total, Securities Held to Maturity, Amortized Cost	30,295		30,295		36,206
Securities Held to Maturity, Estimated Fair Value [Abstract]
Total, Securities Held to Maturity, Estimated Fair Value	33,604		33,604		40,390
Residential Mortgage Pass Through Securities Guaranteed by GNMA [Member]
Fair value [Abstract]
Estimated Fair Value, Less than 12 Months	38,003		38,003
Estimated Fair Value, 12 Months or More	0		0
Estimated Fair Value, Total	38,003		38,003
Gross unrealized losses [Abstract]
Gross Unrealized (Losses), Less than 12 Months	(1,694)		(1,694)
Gross Unrealized (Losses), 12 Months or More	0		0
Gross Unrealized (Losses), Total	(1,694)		(1,694)
Securities Held to Maturity, Amortized Cost [Abstract]
Total, Securities Held to Maturity, Amortized Cost	2,547		2,547		3,245
Securities Held to Maturity, Estimated Fair Value [Abstract]
Total, Securities Held to Maturity, Estimated Fair Value	2,686		2,686		3,472
Residential Mortgage Pass Through Securities Issued by FNMA and FHLMC [Member]
Fair value [Abstract]
Estimated Fair Value, Less than 12 Months	60,115		60,115
Estimated Fair Value, 12 Months or More	0		0
Estimated Fair Value, Total	60,115		60,115
Gross unrealized losses [Abstract]
Gross Unrealized (Losses), Less than 12 Months	(1,751)		(1,751)
Gross Unrealized (Losses), 12 Months or More	0		0
Gross Unrealized (Losses), Total	(1,751)		(1,751)
Securities Held to Maturity, Amortized Cost [Abstract]
Total, Securities Held to Maturity, Amortized Cost	567		567		572
Securities Held to Maturity, Estimated Fair Value [Abstract]
Total, Securities Held to Maturity, Estimated Fair Value	602		602		624
Other Residential Mortgage Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Fair value [Abstract]
Estimated Fair Value, Less than 12 Months	1,336,865		1,336,865		3,055
Estimated Fair Value, 12 Months or More	0		0		0
Estimated Fair Value, Total	1,336,865		1,336,865		3,055
Gross unrealized losses [Abstract]
Gross Unrealized (Losses), Less than 12 Months	(27,302)		(27,302)		(20)
Gross Unrealized (Losses), 12 Months or More	0		0		0
Gross Unrealized (Losses), Total	(27,302)		(27,302)		(20)
Securities Held to Maturity, Amortized Cost [Abstract]
Total, Securities Held to Maturity, Amortized Cost	0		0		0
Securities Held to Maturity, Estimated Fair Value [Abstract]
Total, Securities Held to Maturity, Estimated Fair Value	0		0		0
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Fair value [Abstract]
Estimated Fair Value, Less than 12 Months	144,284		144,284		0
Estimated Fair Value, 12 Months or More	0		0		16,339
Estimated Fair Value, Total	144,284		144,284		16,339
Gross unrealized losses [Abstract]
Gross Unrealized (Losses), Less than 12 Months	(5,750)		(5,750)		0
Gross Unrealized (Losses), 12 Months or More	0		0		(154)
Gross Unrealized (Losses), Total	(5,750)		(5,750)		(154)
Securities Held to Maturity, Amortized Cost [Abstract]
Total, Securities Held to Maturity, Amortized Cost	36,929		36,929		2,165
Securities Held to Maturity, Estimated Fair Value [Abstract]
Total, Securities Held to Maturity, Estimated Fair Value	35,041		35,041		2,402
Asset-backed Securities [Member]
Fair value [Abstract]
Estimated Fair Value, Less than 12 Months					16,412
Estimated Fair Value, 12 Months or More					0
Estimated Fair Value, Total					16,412
Gross unrealized losses [Abstract]
Gross Unrealized (Losses), Less than 12 Months					(21)
Gross Unrealized (Losses), 12 Months or More					0
Gross Unrealized (Losses), Total					(21)
Securities Held to Maturity, Amortized Cost [Abstract]
Total, Securities Held to Maturity, Amortized Cost	0		0		0
Securities Held to Maturity, Estimated Fair Value [Abstract]
Total, Securities Held to Maturity, Estimated Fair Value	$ 0		$ 0		$ 0