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, 2018	Dec. 31, 2017
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 14,548,856	$ 12,663,455
Held-to-maturity securities	14,229,439	12,548,280
U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	707,574	661,545
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	7,889,876	6,266,295
Agency-issued commercial mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,781,840	1,844,116
US States and Political Subdivisions Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	1,021,814	744,213
Held-to-maturity Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	14,548,856
Held-to-maturity securities	14,229,439
One Year or Less - Amortized Cost	8,903
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	8,872
After One Year to Five Years - Amortized Cost	394,549
After Five Years to Ten Years - Amortized Cost	1,503,769
After Ten Years - Amortized Cost	12,604,013
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	12,328,222
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	388,576
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	1,541,391
Held-to-maturity Securities [Member] | U.S. agency debentures
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	714,008
Held-to-maturity securities	707,574
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	122,092
After Five Years to Ten Years - Amortized Cost	586,285
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	121,289
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	591,916
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	8,043,443
Held-to-maturity securities	7,889,876
One Year or Less - Amortized Cost	718
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	715
After One Year to Five Years - Amortized Cost	198,545
After Five Years to Ten Years - Amortized Cost	266,194
After Ten Years - Amortized Cost	7,570,748
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	7,428,541
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	194,426
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	273,432
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	422,211
After Ten Years - Amortized Cost	2,227,048
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	2,161,686
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	438,437
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	243,749
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	244,438
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	1,833,961
Held-to-maturity securities	1,781,840
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	1,833,961
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	1,781,840
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	1,048,210
Held-to-maturity securities	1,021,814
One Year or Less - Amortized Cost	8,185
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value	8,157
After One Year to Five Years - Amortized Cost	73,912
After Five Years to Ten Years - Amortized Cost	229,079
After Ten Years - Amortized Cost	728,507
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value	711,717
Held-to-maturity Securities, Debt Maturities, Year Two Through Five, Fair Value	72,861
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount	$ 237,606
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	$ 244,438	256,481
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	243,749
Held-to-maturity securities	244,438
Fixed rate | Residential mortgage-backed securities | Agency-issued collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity securities	2,583,897	$ 2,775,630
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	2,665,485
Held-to-maturity securities	$ 2,583,897