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	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 11,055,006	$ 8,426,998
Held-to-maturity securities	11,023,415	8,376,138
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	665,888	629,087
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	5,163,349	2,878,572
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,544,548	1,144,962
US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	380,608	79,903
Held-to-maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	11,055,006
Held-to-maturity securities	11,023,415
One Year or Less - Amortized Cost	8,298
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	8,271
After One Year to Five Years - Amortized Cost	429,487
After Five Years to Ten Years - Amortized Cost	1,214,637
After Ten Years - Amortized Cost	9,400,697
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	9,370,452
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	430,055
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	1,216,524
Held-to-maturity Securities [Member] | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	660,193
Held-to-maturity securities	665,888
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	102,581
After Five Years to Ten Years - Amortized Cost	562,490
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	103,398
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	557,612
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	5,164,701
Held-to-maturity securities	5,163,349
One Year or Less - Amortized Cost	738
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	731
After One Year to Five Years - Amortized Cost	255,275
After Five Years to Ten Years - Amortized Cost	61,270
After Ten Years - Amortized Cost	4,847,150
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	4,846,220
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	255,128
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	61,538
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	431,500
After Ten Years - Amortized Cost	2,587,556
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	2,567,360
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	437,865
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	269,495
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	270,162
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	1,554,220
Held-to-maturity securities	1,544,548
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	1,554,220
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	1,544,548
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	380,976
Held-to-maturity securities	380,608
One Year or Less - Amortized Cost	7,560
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	7,540
After One Year to Five Years - Amortized Cost	71,631
After Five Years to Ten Years - Amortized Cost	159,377
After Ten Years - Amortized Cost	142,276
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	142,162
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	71,529
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	$ 159,509
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	$ 270,162	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	269,495
Held-to-maturity securities	270,162
Fixed rate | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	2,998,860	$ 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	3,025,421
Held-to-maturity securities	$ 2,998,860