Equity (Details)						3 Months Ended												12 Months Ended			48 Months Ended
	Jun. 12, 2019 USD ($)	May 13, 2019 USD ($) $ / shares shares	Jun. 20, 2014 $ / shares shares	Mar. 25, 2014	Jan. 17, 2013 $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares shares	Sep. 30, 2018 $ / shares	Jun. 30, 2018 $ / shares	Mar. 31, 2018 $ / shares	Dec. 31, 2017 $ / shares	Sep. 30, 2017 $ / shares	Jun. 30, 2017 $ / shares	Mar. 31, 2017 $ / shares	Dec. 31, 2019 USD ($) security director $ / shares shares	Dec. 31, 2018 USD ($) security $ / shares shares	Dec. 31, 2017 USD ($) security $ / shares shares	Dec. 31, 2019 USD ($) shares	Mar. 14, 2019 USD ($)	Sep. 30, 2015 USD ($)
Class of Stock [Line Items]
Quarterly dividend on all classes of common stock (in dollars per share) | $ / shares						$ 0.70	$ 0.70	$ 0.70	$ 0.70	$ 0.58	$ 0.58	$ 0.58	$ 0.58	$ 0.36	$ 0.36	$ 0.36	$ 0.36
Repurchase of stock																		$ 75,000,000	$ 0	$ 0
Deferred issuance costs						$ 0	$ 0	$ 1,956,000	$ 0									$ 1,956,000	$ 0	$ 0
Expiration period of SARs (in years)																		10 years
Exercise price of SARs granted (in dollars per share) | $ / shares																		$ 82.76	$ 86.15	$ 60.84
Tax benefit from exercise of stock options																		$ 1,000,000.0	$ 1,500,000	$ 2,600,000
Excess tax benefits related to stock-based awards																		449,000	946,000	860,000
Net decrease to additional paid-in capital related to stock-based compensation awards																		$ (1,800,000)	(2,700,000)	(2,600,000)
Number of board of directors electing to receive shares | director																		15
SARs outstanding (in shares) | shares						98,836												98,836			98,836
Intrinsic value of options, SARs, and non-vested restricted stock outstanding, exercisable, and vested or expected to vest						$ 8,900,000				$ 7,900,000								$ 8,900,000	7,900,000		$ 8,900,000
Total intrinsic value of options and SARs exercised																		1,900,000	$ 3,000,000.0	$ 3,800,000
Total unrecognized compensation cost related to non-vested SARs and restricted stock awards						2,700,000												$ 2,700,000			2,700,000
Compensation cost recognition period (in years)																		1 year 9 months 18 days
Weighted-average grant date fair values of options, SARs, and restricted stock awards granted (in dollars per share) | $ / shares																		$ 58.27	$ 69.38	$ 44.93
Compensation expense																		$ 2,300,000	$ 2,500,000	$ 2,700,000
Weighted-average grant date fair value of the restricted stock awarded (in dollars per share) | $ / shares																		$ 80.51	$ 84.03	$ 59.79
Capital required for capital adequacy						618,800,000				545,000,000.0								$ 618,800,000	$ 545,000,000.0		618,800,000
Capital						815,400,000				727,600,000								815,400,000	727,600,000		815,400,000
Excess capital						$ 196,600,000				$ 182,600,000								$ 196,600,000	$ 182,600,000		$ 196,600,000
Employee Stock Option
Class of Stock [Line Items]
SARs outstanding (in shares) | shares						0												0			0
Restricted Stock | Minimum
Class of Stock [Line Items]
Vesting period of awards																		1 year
Restricted Stock | Maximum
Class of Stock [Line Items]
Vesting period of awards																		3 years
5.700% Non-Cumulative Preferred Stock, Series D
Class of Stock [Line Items]
Preferred stock, shares issued (in shares) | shares		4,000,000				4,000,000				4,000,000								4,000,000	4,000,000		4,000,000
Dividend rate (as a percent)		5.70%		6.875%
Liquidation value (in dollars per share) | $ / shares		$ 25.00			$ 25.00
Aggregate outstanding		$ 100,000,000.0
Repurchase of stock	$ 75,000,000.0
Issuance costs																		$ 3,300,000
Dividends on preferred stock (in dollars per share) | $ / shares						$ 0.35625	$ 0.35625
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares issued (in shares) | shares						3,000,000				3,000,000								3,000,000	3,000,000		3,000,000
Deferred issuance costs	$ 2,000,000.0
Issuance costs																		$ 1,900,000
Dividends on preferred stock (in dollars per share) | $ / shares								$ 0.4297	$ 0.4297	$ 0.4297	0.4297	0.4297	0.4297	0.4297	0.4297	0.4297	0.4297
Series B Preferred Stock | Second Quarter 2019 Not Including April 17, 2019 to and including June 12, 2019
Class of Stock [Line Items]
Dividends on preferred stock (in dollars per share) | $ / shares								0.2626
Class C non-voting common stock
Class of Stock [Line Items]
Stock authorized for repurchase																						$ 10,000,000.0	$ 25,000,000.0
Shares repurchased (in shares) | shares																		668,000
Shares repurchased																					$ 19,600,000
Class C shares issued (in shares) | shares																		768	174	698
Fair value of Class C stock issued																		$ 60,000	$ 14,000	$ 41,000
Number of board of directors electing to receive shares | security																		4	4	4
5.875% Non-Cumulative Preferred Stock, Series A
Class of Stock [Line Items]
Preferred stock, shares issued (in shares) | shares					2,400,000	2,400,000				2,400,000								2,400,000	2,400,000		2,400,000
Dividend rate (as a percent)					5.875%
Liquidation value (in dollars per share) | $ / shares					$ 25.00
Issuance costs																		$ 1,700,000
Dividends on preferred stock (in dollars per share) | $ / shares						$ 0.3672	0.3672	0.3672	0.3672	$ 0.3672	0.3672	0.3672	0.3672	0.3672	0.3672	0.3672	0.3672
6.000% Fixed-to-Floating Rate Non-Cumulative Preferred Stock, Series C
Class of Stock [Line Items]
Preferred stock, shares issued (in shares) | shares			3,000,000			3,000,000				3,000,000								3,000,000	3,000,000		3,000,000
Dividend rate (as a percent)			6.00%
Liquidation value (in dollars per share) | $ / shares			$ 25.00
Issuance costs																		$ 1,600,000
Dividends on preferred stock (in dollars per share) | $ / shares						$ 0.3750	$ 0.3750	$ 0.3750	$ 0.3750	$ 0.3750	$ 0.3750	$ 0.3750	$ 0.3750	$ 0.3750	$ 0.3750	$ 0.3750	$ 0.3750
6.000% Fixed-to-Floating Rate Non-Cumulative Preferred Stock, Series C | July 17, 2024, thereafter
Class of Stock [Line Items]
Dividend variable rate (as a percent)																		3.26%