Equity - Narrative (Details) - USD ($)						1 Months Ended	3 Months Ended		12 Months Ended
	Sep. 19, 2020	Aug. 20, 2020	May 20, 2020	May 13, 2019	Jun. 20, 2014	May 31, 2020	Mar. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 14, 2019	Dec. 31, 2017	Sep. 30, 2015
Class of Stock [Line Items]
Quarterly dividend on all classes of common stock (in dollars per share)									$ 0.80	$ 0.70	$ 0.58
Deferred issuance costs								$ 1,956,000	$ 1,667,000	$ 1,956,000	$ 0
Share expiration period									10 years
SARs exercise price (in dollars per share)										$ 82.76	$ 86.15
Excess tax benefits related to stock-based awards									$ (440,000)	$ 449,000	$ 946,000
Decrease in additional paid-in capital									$ (600,000)	(1,800,000)	(2,700,000)
Stock options outstanding									116,417
Intrinsic value of outstanding, exercisable and vested or excepted to vest restricted stock								8,900,000	$ 8,500,000	8,900,000
Intrinsic value of shares exercised									700,000	$ 1,900,000	$ 3,000,000.0
Unrecognized compensation cost									$ 2,400,000
Weighted-average period of recognition									1 year 9 months 18 days
Weighted-average grant date fair value (in dollars per share)									$ 45.91	$ 58.27	$ 69.38
Compensation expense									$ 4,100,000	$ 2,300,000	$ 2,500,000
Weighted-average grant date fair value of the restricted stock awarded (in dollars per share)									$ 66.02	$ 80.51	$ 84.03
Capital required for capital adequacy								618,800,000	$ 680,900,000	$ 618,800,000
Capital								815,400,000	1,000,000,000.0	815,400,000
Excess capital								$ 196,600,000	$ 325,500,000	$ 196,600,000
5.250% Non-Cumulative Preferred Stock, Series F
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)		4,800,000						4,800,000	4,800,000	4,800,000
Preferred stock, dividend rate		5.25%
Liquidation value (in dollars per share)		$ 25.00
Preferred stock		$ 120,000,000.0						$ 0	$ 116,160,000	$ 0
Payments of stock issuance costs		$ 3,800,000							$ 3,839,902
Series A
Class of Stock [Line Items]
Stock authorized for repurchase	$ 60,000,000.0
Preferred stock, shares issued (in shares)								2,400,000	2,400,000	2,400,000
Preferred stock, dividend rate	5.875%
Preferred stock								$ 58,333,000	$ 0	$ 58,333,000
Deferred issuance costs	$ 1,700,000
Series E
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			3,180,000			3,180,000		3,180,000	3,180,000	3,180,000
Preferred stock, dividend rate			5.75%			5.75%
Liquidation value (in dollars per share)			$ 25.00			$ 25.00
Preferred stock						$ 79,500,000		$ 0	$ 77,003,000	$ 0
Payments of stock issuance costs			$ 2,496,750			$ 2,500,000
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)							4,000		673,000
Shares repurchased							$ 200,000		$ 19,800,000
6.000% Fixed-to-Floating Rate Non-Cumulative Preferred Stock, Series C
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)					3,000,000			3,000,000	3,000,000	3,000,000
Preferred stock, dividend rate					6.00%
Liquidation value (in dollars per share)					$ 25.00
Preferred stock								$ 73,382,000	$ 73,382,000	$ 73,382,000
Payments of stock issuance costs					$ 1,618,583
5.700% Non-Cumulative Preferred Stock, Series D
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)				4,000,000				4,000,000	4,000,000	4,000,000
Preferred stock, dividend rate				5.70%
Liquidation value (in dollars per share)				$ 25.00
Preferred stock								$ 96,659,000	$ 96,659,000	$ 96,659,000
Payments of stock issuance costs				$ 3,340,456
Minimum
Class of Stock [Line Items]
SARs exercise price (in dollars per share)									$ 72.26
Maximum
Class of Stock [Line Items]
SARs exercise price (in dollars per share)									$ 75.16
Stock option
Class of Stock [Line Items]
Stock options outstanding									0
Restricted Stock | Minimum
Class of Stock [Line Items]
Vesting period									1 year
Restricted Stock | Maximum
Class of Stock [Line Items]
Vesting period									3 years
SARs
Class of Stock [Line Items]
Tax benefit from exercise of SARs									$ 500,000	1,000,000.0	$ 1,500,000
Excess tax benefits related to stock-based awards									$ 8,900	$ 400,000
Stock options outstanding								98,836	116,417	98,836	124,960		163,272