Securities Available for Sale and Held to Maturity (1) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available for sale and Held to maturity Securities [Line Items]
Reclassification of Securities available for sale to securities held to maturity						$ 1,099,000
Net unrealized holding loss on AFS Securities at date of transfer						46,600
Net unrealized holding losses on AFS Securities, net of tax at date of transfer						$ 28,800
Net unamortized, unrealized loss on transfer of securities	$ 35,600		$ 35,600
Net unamortized, unrealized loss on transfer of securities, net of tax	22,000		22,000
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	206,281		206,281		$ 590,965
Estimated Fair Value, 12 Months or More	294,855		294,855		522,230
Estimated Fair Value, Total	501,136		501,136		1,113,195
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(800)		(2,663)
Gross Unrealized (Losses), 12 Months or More			(5,026)		(11,909)
Gross Unrealized (Losses), Total			(5,826)		(14,572)
Gain (Loss) on Investments [Line Items]
Other-than-temporary impairments			0	$ 0
Securities Available for Sale, Amortized Cost [Abstract]
Due in one year or less	17,392		17,392
Due after one year through five years	148,869		148,869
Due after five years through ten years	23,380		23,380
Due after ten years	73,389		73,389
Total amortized cost, before mortgage-backed securities	263,030		263,030
Mortgage-backed securities	2,083,520		2,083,520
Securities available for sale (at amortized cost)	2,346,550		2,346,550
Securities Available for Sale, Estimated Fair Value [Abstract]
Due in one year or less	17,525		17,525
Due after one year through five years	154,170		154,170
Due after five years through ten years	23,794		23,794
Due after ten years	73,260		73,260
Total fair value, before mortgage-backed securities	268,749		268,749
Mortgage-backed securities	2,114,073		2,114,073
Securities available for sale (at fair value)	2,382,822		2,382,822		2,374,567
Securities Held to Maturity, Amortized Cost [Abstract]
Due in one year or less	825		825
Due after one year through five years	9,597		9,597
Due after five years through ten years	135,959		135,959
Due after ten years	11,858		11,858
Total amortized cost, before mortgage-backed securities	158,239		158,239
Mortgage-backed securities	1,020,201		1,020,201
Amortized Cost	1,178,440		1,178,440		1,170,685
Securities Held to Maturity, Estimated Fair Value [Abstract]
Due in one year or less	835		835
Due after one year through five years	9,954		9,954
Due after five years through ten years	142,132		142,132
Due after ten years	12,940		12,940
Total fair value, before mortgage-backed securities	165,861		165,861
Mortgage-backed securities	1,037,207		1,037,207
Securities held to maturity (at fair value)	1,203,068		1,203,068		1,182,846
Available for Sale [Member]
Gains (losses) on Investments [Abstract]
Proceeds from calls and sales of securities	0	$ 30,540	0	56,815
Gross realized gains	0	67	0	456
Gross realized (losses)	0	$ (156)	0	$ (156)
U.S. Government Agency Obligations Issued by U.S. Government Agencies [Member]
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	9,150		9,150		19,220
Estimated Fair Value, 12 Months or More	32,228		32,228		32,656
Estimated Fair Value, Total	41,378		41,378		51,876
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(18)		(115)
Gross Unrealized (Losses), 12 Months or More			(409)		(711)
Gross Unrealized (Losses), Total			(427)		(826)
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	0		0		0
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	0		0		0
U.S. Government Agency Obligations Issued by U.S. Government Sponsored Agencies [Member]
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months					9,948
Estimated Fair Value, 12 Months or More					9,956
Estimated Fair Value, Total					19,904
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months					(52)
Gross Unrealized (Losses), 12 Months or More					(25)
Gross Unrealized (Losses), Total					(77)
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	101,578		101,578		100,971
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	105,958		105,958		103,602
Obligations of States and Political Subdivisions [Member]
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	1,275		1,275		8,431
Estimated Fair Value, 12 Months or More	2,860		2,860		3,800
Estimated Fair Value, Total	4,135		4,135		12,231
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(4)		(22)
Gross Unrealized (Losses), 12 Months or More			(9)		(46)
Gross Unrealized (Losses), Total			(13)		(68)
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	56,661		56,661		63,505
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	59,903		59,903		66,903
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	19,671		19,671		7,199
Estimated Fair Value, 12 Months or More	0		0		647
Estimated Fair Value, Total	19,671		19,671		7,846
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(123)		(22)
Gross Unrealized (Losses), 12 Months or More			0		(8)
Gross Unrealized (Losses), Total			(123)		(30)
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	17,783		17,783		19,115
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	18,199		18,199		19,565
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	2,485		2,485
Estimated Fair Value, 12 Months or More	0		0
Estimated Fair Value, Total	2,485		2,485
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]
Amortized Cost	10,669		10,669		11,437
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	11,126		11,126		11,908
Other Residential Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	161,853		161,853		537,033
Estimated Fair Value, 12 Months or More	248,585		248,585		395,342
Estimated Fair Value, Total	410,438		410,438		932,375
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(544)		(2,449)
Gross Unrealized (Losses), 12 Months or More			(4,544)		(10,031)
Gross Unrealized (Losses), Total			(5,088)		(12,480)
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	808,763		808,763		834,176
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	820,644		820,644		838,700
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	11,847		11,847		9,134
Estimated Fair Value, 12 Months or More	11,182		11,182		79,829
Estimated Fair Value, Total	23,029		23,029		88,963
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(90)		(3)
Gross Unrealized (Losses), 12 Months or More			(64)		(1,088)
Gross Unrealized (Losses), Total			(154)		(1,091)
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	182,986		182,986		141,481
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	187,238		187,238		142,168
Asset-Backed Securities and Structured Financial Products [Member]
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	0		0		0
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	$ 0		$ 0		$ 0