Equity (Details) - USD ($) $ / shares in Units, shares in Thousands	1 Months Ended	3 Months Ended						57 Months Ended
	May 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Mar. 14, 2019	Dec. 31, 2015
Class of Stock [Line Items]
Quarterly dividend on all classes of common stock (in dollars per share)		$ 0.80	$ 0.80	$ 0.70	$ 0.70	$ 0.70	$ 0.70
Capital required for capital adequacy		$ 667,700,000		$ 618,800,000				$ 667,700,000
Capital		915,600,000		815,400,000				915,600,000
Excess capital		$ 247,900,000		$ 196,600,000				$ 247,900,000
Series E Preferred Stock
Class of Stock [Line Items]
Number of shares issued (in shares)	3,200
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					673
Shares repurchased			$ 200,000					$ 19,800,000