Fair Value of Financial Instruments - Summary of Estimated Fair Values of Financial Instruments not Carried at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 15,188,236	$ 12,548,280
Net loans	28,057,377	22,851,292
Time deposits	50,726	47,146
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,571,539	2,923,075
Held-to-maturity securities	15,487,442	12,663,455
Non-marketable securities not measured at net asset value	131,453	120,019
Non-marketable securities measured at net asset value	151,247	228,399
FHLB and FRB stock	58,878	60,020
Non-maturity deposits	49,278,174	44,206,929
Time deposits	50,726	47,146
Commitments to extend credit	0	0
Carrying Amount | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	631,412	1,033,730
Carrying Amount | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	347,639	347,303
Carrying Amount | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	348,826	348,189
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,571,539	2,923,075
Held-to-maturity securities	15,188,236	12,548,280
Non-marketable securities not measured at net asset value	131,453	126,345
Non-marketable securities measured at net asset value	151,247	331,496
FHLB and FRB stock	58,878	60,020
Non-maturity deposits	49,278,174	44,206,929
Time deposits	50,337	46,885
Commitments to extend credit	22,930	22,208
Estimated Fair Value | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	631,412	1,033,730
Estimated Fair Value | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	336,088	352,058
Estimated Fair Value | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	361,281	374,483
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	3,571,539	2,923,075
Held-to-maturity securities	0	0
Non-marketable securities not measured at net asset value	0	0
FHLB and FRB stock	0	0
Non-maturity deposits	49,278,174	44,206,929
Time deposits	0	0
Commitments to extend credit	0	0
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0	1,033,730
Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1) | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	0	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Held-to-maturity securities	15,188,236	12,548,280
Non-marketable securities not measured at net asset value	0	0
FHLB and FRB stock	0	0
Non-maturity deposits	0	0
Time deposits	50,337	46,885
Commitments to extend credit	0	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	631,412	0
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | 3.50% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	336,088	352,058
Estimated Fair Value | Significant Other Observable Inputs (Level 2) | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	361,281	374,483
Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Held-to-maturity securities	0	0
Non-marketable securities not measured at net asset value	131,453	126,345
FHLB and FRB stock	58,878	60,020
Non-maturity deposits	0	0
Time deposits	0	0
Commitments to extend credit	22,930	22,208
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | Other short-term borrowings
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term borrowings	0	0
Estimated Fair Value | Significant Unobservable Inputs (Level 3) | 5.375% Senior Notes
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Senior Notes	0	0
Commercial loans | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	25,043,671	20,238,247
Commercial loans | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	25,463,968	20,520,623
Commercial loans | Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	0	0
Commercial loans | Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	0	0
Commercial loans | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	25,463,968	20,520,623
Consumer loans | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	3,013,706	2,613,045
Consumer loans | Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	3,064,093	2,593,538
Consumer loans | Estimated Fair Value | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	0	0
Consumer loans | Estimated Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	0	0
Consumer loans | Estimated Fair Value | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Net loans	$ 3,064,093	$ 2,593,538