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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 12,663,455	$ 8,426,998
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	12,663,455	8,426,998
Held-to-maturity Securities, Fair Value	12,548,280	8,376,138
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	12,663,455
Held-to-maturity Securities, Fair Value	12,548,280
One Year or Less - Amortized Cost	7,801
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	7,777
One Year to Five Years - Amortized Cost	402,547
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	400,149
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	1,310,124
After Five Years to Ten Years - Amortized Cost	1,298,829
After Ten Years - Amortized Cost	10,942,983
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	$ 10,841,525
Held-to-maturity securities | Lower Limit
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	10 years
Held-to-maturity securities | Upper Limit
Schedule of Held-to-maturity Securities [Line Items]
Mortgage-backed securities contractual maturities (in years)	30 years
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	$ 659,979	622,445
Held-to-maturity Securities, Fair Value	661,545	629,087
U.S. agency debentures | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	659,979
Held-to-maturity Securities, Fair Value	661,545
One Year or Less - Amortized Cost	0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
One Year to Five Years - Amortized Cost	102,496
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	102,739
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	557,483
After Five Years to Ten Years - Amortized Cost	558,806
After Ten Years - Amortized Cost	0
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	0
Agency-issued mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	6,304,969	2,896,179
Held-to-maturity Securities, Fair Value	6,266,295	2,878,572
Agency-issued mortgage-backed securities | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	6,304,969
Held-to-maturity Securities, Fair Value	6,266,295
One Year or Less - Amortized Cost	728
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	723
One Year to Five Years - Amortized Cost	226,997
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	225,149
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	56,380
After Five Years to Ten Years - Amortized Cost	55,697
After Ten Years - Amortized Cost	6,020,864
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	5,984,726
Residential mortgage-backed securities | Held-to-maturity securities | Agency-issued collateralized mortgage obligations | Fixed rate
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	2,829,979
One Year or Less - Amortized Cost	0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
One Year to Five Years - Amortized Cost	0
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	462,533
After Five Years to Ten Years - Amortized Cost	451,069
After Ten Years - Amortized Cost	2,367,446
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	2,324,561
Residential mortgage-backed securities | Held-to-maturity securities | Agency-issued collateralized mortgage obligations | Variable rate
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	255,782
One Year or Less - Amortized Cost	0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
One Year to Five Years - Amortized Cost	0
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
After Five Years to Ten Years - Amortized Cost	0
After Ten Years - Amortized Cost	255,782
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	256,481
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	1,868,985	1,151,363
Held-to-maturity Securities, Fair Value	1,844,116	1,144,962
Agency-issued commercial mortgage-backed securities | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	1,868,985
Held-to-maturity Securities, Fair Value	1,844,116
One Year or Less - Amortized Cost	0
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	0
One Year to Five Years - Amortized Cost	0
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
After Five Years to Ten Years - Amortized Cost	0
After Ten Years - Amortized Cost	1,868,985
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	1,844,116
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	743,761	81,748
Held-to-maturity Securities, Fair Value	744,213	79,903
Municipal bonds and notes | Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	743,761
Held-to-maturity Securities, Fair Value	744,213
One Year or Less - Amortized Cost	7,073
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	7,054
One Year to Five Years - Amortized Cost	73,054
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	72,261
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	233,728
After Five Years to Ten Years - Amortized Cost	233,257
After Ten Years - Amortized Cost	429,906
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	431,641
Variable rate | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	255,782	312,665
Held-to-maturity Securities, Fair Value	256,481	311,502
Variable rate | Residential mortgage-backed securities | Held-to-maturity securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Fair Value	256,481
Fixed rate | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	2,829,979	3,362,598
Held-to-maturity Securities, Fair Value	2,775,630	$ 3,332,112
Fixed rate | Residential mortgage-backed securities | Held-to-maturity securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Fair Value	$ 2,775,630