Equity (Details) - USD ($) $ / shares in Units, shares in Thousands	3 Months Ended					54 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Mar. 31, 2020	Mar. 14, 2019	Sep. 30, 2015
Class of Stock [Line Items]
Quarterly dividend on all classes of common stock (in dollars per share)	$ 0.80	$ 0.70	$ 0.70	$ 0.70	$ 0.70
Capital required for capital adequacy	$ 649,300,000	$ 618,800,000				$ 649,300,000
Capital	815,100,000	815,400,000				815,100,000
Excess capital	$ 165,800,000	$ 196,600,000				$ 165,800,000
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