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	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 9,938,371	$ 8,426,998
Held-to-maturity securities	9,910,504	8,376,138
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	647,453	629,087
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	4,168,013	2,878,572
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,379,695	1,144,962
US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	216,031	79,903
Held-to-maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	9,938,371
Held-to-maturity securities	9,910,504
One Year or Less - Amortized Cost	7,609
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	7,590
After One Year to Five Years - Amortized Cost	434,712
After Five Years to Ten Years - Amortized Cost	898,444
After Ten Years - Amortized Cost	8,601,254
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	8,570,191
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	434,279
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	894,796
Held-to-maturity Securities [Member] | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	641,191
Held-to-maturity securities	647,453
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	75,321
After Five Years to Ten Years - Amortized Cost	571,418
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	76,035
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	565,870
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	4,170,585
Held-to-maturity securities	4,168,013
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	288,094
After Five Years to Ten Years - Amortized Cost	66,539
After Ten Years - Amortized Cost	3,815,542
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	3,814,245
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	287,229
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	66,949
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	149,193
After Ten Years - Amortized Cost	3,089,703
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	3,066,076
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	150,940
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	283,519
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	284,043
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	1,386,455
Held-to-maturity securities	1,379,695
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	1,386,455
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	1,379,695
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	215,978
Held-to-maturity securities	216,031
One Year or Less - Amortized Cost	7,609
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	7,590
After One Year to Five Years - Amortized Cost	71,297
After Five Years to Ten Years - Amortized Cost	111,294
After Ten Years - Amortized Cost	26,035
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	26,132
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	71,015
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	$ 111,037
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	$ 284,043	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	283,519
Held-to-maturity securities	284,043
Fixed rate | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	3,215,269	$ 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	3,240,643
Held-to-maturity securities	$ 3,215,269