Equity (Details) - USD ($)		1 Months Ended	3 Months Ended							9 Months Ended		57 Months Ended
	Sep. 19, 2020	May 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Mar. 14, 2019	Mar. 31, 2016
Class of Stock [Line Items]
Loss on retirement of preferred stock			$ 1,667,000				$ 0	$ 1,956,000		$ 1,667,000	$ 1,956,000
Quarterly dividend on all classes of common stock (in dollars per share)			$ 0.80	$ 0.80	$ 0.80	$ 0.70	$ 0.70	$ 0.70	$ 0.70
Capital required for capital adequacy			$ 670,000,000.0			$ 618,800,000				670,000,000.0
Capital			984,200,000			815,400,000				984,200,000
Excess capital			$ 314,200,000			$ 196,600,000				$ 314,200,000
5.250% Non-Cumulative Preferred Stock, Series F
Class of Stock [Line Items]
Number of shares issued (in shares)		4,800,000
Preferred stock, dividend rate (as a percent)		5.25%
Preferred stock, par value (in dollars per share)		$ 25.00	$ 25			$ 25				$ 25
Payments of stock issuance costs		$ 3,800,000
Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock, dividend rate (as a percent)	5.875%
Preferred stock, par value (in dollars per share)			25			25				25
Stock authorized for repurchase	$ 60,000,000.0
Loss on retirement of preferred stock	$ 1,700,000
Series E Preferred Stock
Class of Stock [Line Items]
Number of shares issued (in shares)		3,180,000
Preferred stock, dividend rate (as a percent)		5.75%
Preferred stock, par value (in dollars per share)		$ 25.00	$ 25			$ 25				$ 25
Payments of stock issuance costs		$ 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