Investment Securities - Summary of Remaining Contractual Principal Maturities and Fully Taxable Equivalent Yields on Debt Securities Classified as Held to Maturity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 8,615,695	$ 8,426,998
Held-to-maturity securities	8,567,817	8,376,138
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	646,579	629,087
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3,009,733	2,878,572
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,282,428	1,144,962
US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	136,982	79,903
Held-to-maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	8,615,695
Held-to-maturity securities	8,567,817
One Year or Less - Amortized Cost	6,168
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	6,159
After One Year to Five Years - Amortized Cost	448,231
After Five Years to Ten Years - Amortized Cost	864,751
After Ten Years - Amortized Cost	7,297,656
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	7,249,918
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	446,989
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	863,640
Held-to-maturity Securities [Member] | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	641,421
Held-to-maturity securities	646,579
One Year or Less - Amortized Cost	0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
After One Year to Five Years - Amortized Cost	75,459
After Five Years to Ten Years - Amortized Cost	570,273
After Ten Years - Amortized Cost	0
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	0
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	76,306
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	565,962
Held-to-maturity Securities [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,020,929
Held-to-maturity securities	3,009,733
One Year or Less - Amortized Cost	0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
After One Year to Five Years - Amortized Cost	317,056
After Five Years to Ten Years - Amortized Cost	76,978
After Ten Years - Amortized Cost	2,626,412
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	2,617,331
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	315,424
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	77,461
Held-to-maturity Securities [Member] | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Fixed rate
Schedule of Held-to-maturity Securities [Line Items]
One Year or Less - Amortized Cost	0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
After One Year to Five Years - Amortized Cost	0
After Five Years to Ten Years - Amortized Cost	158,587
After Ten Years - Amortized Cost	3,062,355
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	3,034,511
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	0
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	160,700
Held-to-maturity Securities [Member] | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations | Variable rate
Schedule of Held-to-maturity Securities [Line Items]
One Year or Less - Amortized Cost	0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
After One Year to Five Years - Amortized Cost	0
After Five Years to Ten Years - Amortized Cost	0
After Ten Years - Amortized Cost	299,257
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	298,997
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	0
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	0
Held-to-maturity Securities [Member] | Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	1,292,744
Held-to-maturity securities	1,282,428
One Year or Less - Amortized Cost	0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
After One Year to Five Years - Amortized Cost	0
After Five Years to Ten Years - Amortized Cost	0
After Ten Years - Amortized Cost	1,292,744
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	1,282,428
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	0
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	0
Held-to-maturity Securities [Member] | US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	138,289
Held-to-maturity securities	136,982
One Year or Less - Amortized Cost	6,168
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	6,159
After One Year to Five Years - Amortized Cost	55,716
After Five Years to Ten Years - Amortized Cost	58,913
After Ten Years - Amortized Cost	16,888
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	16,651
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	55,259
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	$ 59,517
Lower Limit | Held-to-maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Upper Limit | Held-to-maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years
Variable rate | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	$ 298,997	311,502
Variable rate | Held-to-maturity Securities [Member] | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	299,257
Held-to-maturity securities	298,997
Fixed rate | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3,193,098	$ 3,332,112
Fixed rate | Held-to-maturity Securities [Member] | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	3,223,055
Held-to-maturity securities	$ 3,193,098