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, 2016	Dec. 31, 2015
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 8,426,998	$ 8,790,963
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	8,426,998	8,790,963
Held-to-maturity Securities, Fair Value	8,376,138	8,758,622
Held-to-maturity securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	8,426,998
Held-to-maturity Securities, Fair Value	8,376,138
One Year or Less - Amortized Cost	4,509
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	4,498
One Year to Five Years - Amortized Cost	345,112
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	344,264
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	853,630
After Five Years to Ten Years - Amortized Cost	856,539
After Ten Years - Amortized Cost	7,223,747
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	$ 7,170,837
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	$ 622,445	545,473
Held-to-maturity Securities, Fair Value	629,087	554,349
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	622,445
Held-to-maturity Securities, Fair Value	629,087
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	47,357
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	47,943
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	575,088
After Five Years to Ten Years - Amortized Cost	581,144
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	2,896,179	2,366,627
Held-to-maturity Securities, Fair Value	2,878,572	2,355,475
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	2,896,179
Held-to-maturity Securities, Fair Value	2,878,572
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	264,607
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	263,759
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	172,472
After Five Years to Ten Years - Amortized Cost	171,290
After Ten Years - Amortized Cost	2,459,100
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	2,443,523
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	3,362,598
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	71,260
After Five Years to Ten Years - Amortized Cost	70,227
After Ten Years - Amortized Cost	3,291,338
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	3,261,885
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	312,665
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	312,665
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	311,502
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	1,151,363	1,214,716
Held-to-maturity Securities, Fair Value	1,144,962	1,214,646
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	1,151,363
Held-to-maturity Securities, Fair Value	1,144,962
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	1,151,363
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	1,144,962
Municipal bonds and notes
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	81,748	67,587
Held-to-maturity Securities, Fair Value	79,903	66,812
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	81,748
Held-to-maturity Securities, Fair Value	79,903
One Year or Less - Amortized Cost	4,509
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	4,498
One Year to Five Years - Amortized Cost	33,148
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	32,562
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	34,810
After Five Years to Ten Years - Amortized Cost	33,878
After Ten Years - Amortized Cost	9,281
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	8,965
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	312,665	370,779
Held-to-maturity Securities, Fair Value	311,502	371,504
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	311,502
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	3,362,598	4,225,781
Held-to-maturity Securities, Fair Value	3,332,112	$ 4,195,836
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	$ 3,332,112