Equity - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	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)	$ 0.88	$ 0.80	$ 0.80	$ 0.80	$ 0.80
Capital required for capital adequacy	$ 677,600,000	$ 680,900,000
Capital	1,000,000,000.0	1,000,000,000.0
Excess capital	$ 348,100,000	$ 325,500,000
Class C non-voting common stock
Class of Stock [Line Items]
Stock authorized for repurchase						$ 10,000,000.0	$ 25,000,000.0
Remaining authorized amount for repurchase					$ 9,800,000
Shares repurchased (in shares)	673,000				4,000
Shares repurchased	$ 19,800,000				$ 200,000