Equity - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended							9 Months Ended
	Sep. 30, 2021	May 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	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	$ 0.88	$ 0.88	$ 0.80	$ 0.80	$ 0.80	$ 0.80
Capital required for capital adequacy	$ 699,600,000		$ 699,600,000			$ 680,900,000				$ 699,600,000
Capital	1,200,000,000		1,200,000,000			1,000,000,000				1,200,000,000
Excess capital	$ 479,500,000		$ 479,500,000			$ 325,500,000				$ 479,500,000
Series G
Class of Stock [Line Items]
Number of shares issued (in shares)		5,000,000
Preferred stock, dividend rate		4.875%
Preferred stock, par value (in dollars per share)	$ 25	$ 25.00	$ 25			$ 25				$ 25
Payments of stock issuance costs		$ 3,700,000
Class C non-voting common stock
Class of Stock [Line Items]
Stock authorized for repurchase											$ 10,000,000	$ 25,000,000
Remaining authorized amount for repurchase					$ 9,800,000
Shares repurchased (in shares)	673,000								4,000	0
Shares repurchased	$ 19,800,000								$ 200,000