Securities Available for Sale and Held to Maturity (1) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	$ 1,077,543		$ 1,077,543		$ 590,965
Estimated Fair Value, 12 Months or More	334,014		334,014		522,230
Estimated Fair Value, Total	1,411,557		1,411,557		1,113,195
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(8,766)		(2,663)
Gross Unrealized (Losses), 12 Months or More			(8,362)		(11,909)
Gross Unrealized (Losses), Total			(17,128)		(14,572)
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	37,300		37,300
Net unamortized, unrealized loss on transfer of securities, net of tax	23,000		23,000
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	17,167		17,167
Due after one year through five years	154,905		154,905
Due after five years through ten years	32,638		32,638
Due after ten years	76,685		76,685
Total amortized cost, before mortgage-backed securities	281,395		281,395
Mortgage-backed securities	2,146,588		2,146,588
Securities available for sale (at amortized cost)	2,427,983		2,427,983
Securities Available for Sale, Estimated Fair Value [Abstract]
Due in one year or less	17,315		17,315
Due after one year through five years	159,763		159,763
Due after five years through ten years	33,349		33,349
Due after ten years	76,780		76,780
Total fair value, before mortgage-backed securities	287,207		287,207
Mortgage-backed securities	2,159,176		2,159,176
Securities available for sale (at fair value)	2,446,383		2,446,383		2,374,567
Securities Held to Maturity, Amortized Cost [Abstract]
Due in one year or less	956		956
Due after one year through five years	9,751		9,751
Due after five years through ten years	135,818		135,818
Due after ten years	11,827		11,827
Total amortized cost, before mortgage-backed securities	158,352		158,352
Mortgage-backed securities	1,031,809		1,031,809
Amortized Cost	1,190,161		1,190,161		1,170,685
Securities Held to Maturity, Estimated Fair Value [Abstract]
Due in one year or less	966		966
Due after one year through five years	10,195		10,195
Due after five years through ten years	140,986		140,986
Due after ten years	12,455		12,455
Total fair value, before mortgage-backed securities	164,602		164,602
Mortgage-backed securities	1,035,615		1,035,615
Securities held to maturity (at fair value)	1,200,217		1,200,217		1,182,846
Available for Sale [Member]
Gains (losses) on Investments [Abstract]
Proceeds from calls and sales of securities	0	$ 0	0	26,274
Gross realized gains	0	0	0	389
Gross realized (losses)	0	$ 0	0	$ 0
U.S. Government Agency Obligations Issued by U.S. Government Agencies [Member]
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	0		0		19,220
Estimated Fair Value, 12 Months or More	33,386		33,386		32,656
Estimated Fair Value, Total	33,386		33,386		51,876
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			0		(115)
Gross Unrealized (Losses), 12 Months or More			(657)		(711)
Gross Unrealized (Losses), Total			(657)		(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	101,374		101,374		100,971
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	104,652		104,652		103,602
Obligations of States and Political Subdivisions [Member]
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	6,046		6,046		8,431
Estimated Fair Value, 12 Months or More	2,849		2,849		3,800
Estimated Fair Value, Total	8,895		8,895		12,231
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(30)		(22)
Gross Unrealized (Losses), 12 Months or More			(27)		(46)
Gross Unrealized (Losses), Total			(57)		(68)
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	56,978		56,978		63,505
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	59,950		59,950		66,903
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	19,542		19,542		7,199
Estimated Fair Value, 12 Months or More	0		0		647
Estimated Fair Value, Total	19,542		19,542		7,846
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(288)		(22)
Gross Unrealized (Losses), 12 Months or More			0		(8)
Gross Unrealized (Losses), Total			(288)		(30)
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	18,265		18,265		19,115
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	18,518		18,518		19,565
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]
Estimated Fair Value [Abstract]
Estimated Fair Value, Less than 12 Months	34,188		34,188
Estimated Fair Value, 12 Months or More	0		0
Estimated Fair Value, Total	34,188		34,188
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(157)
Gross Unrealized (Losses), 12 Months or More			0
Gross Unrealized (Losses), Total			(157)
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	10,965		10,965		11,437
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	11,285		11,285		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	846,185		846,185		537,033
Estimated Fair Value, 12 Months or More	271,538		271,538		395,342
Estimated Fair Value, Total	1,117,723		1,117,723		932,375
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(6,470)		(2,449)
Gross Unrealized (Losses), 12 Months or More			(7,520)		(10,031)
Gross Unrealized (Losses), Total			(13,990)		(12,480)
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	838,989		838,989		834,176
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	841,095		841,095		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	171,582		171,582		9,134
Estimated Fair Value, 12 Months or More	26,241		26,241		79,829
Estimated Fair Value, Total	197,823		197,823		88,963
Gross Unrealized (Losses) [Abstract]
Gross Unrealized (Losses), Less than 12 Months			(1,821)		(3)
Gross Unrealized (Losses), 12 Months or More			(158)		(1,088)
Gross Unrealized (Losses), Total			(1,979)		(1,091)
Securities Held to Maturity, Amortized Cost [Abstract]
Amortized Cost	163,590		163,590		141,481
Securities Held to Maturity, Estimated Fair Value [Abstract]
Securities held to maturity (at fair value)	164,717		164,717		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