Equity (Details) - USD ($)		3 Months Ended
	Sep. 30, 2023	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Mar. 31, 2022	Mar. 31, 2020	Mar. 31, 2021	Mar. 14, 2019	Sep. 30, 2015
Class of Stock [Line Items]
Common stock dividends per common share (in dollars per share)		$ 1.10	$ 1.10	$ 1.10	$ 0.95	$ 0.95	$ 0.95	$ 0.95
Capital required for capital adequacy	$ 840,100,000	$ 840,100,000			$ 805,900,000
Capital	1,400,000,000	1,400,000,000			1,300,000,000
Excess capital	$ 581,100,000	$ 581,100,000			$ 516,900,000
Class C non-voting common stock
Class of Stock [Line Items]
Stock authorized for repurchase											$ 10,000,000	$ 25,000,000
Shares repurchased (in shares)	673,000	0							4,000
Shares repurchased	$ 19,800,000								$ 200,000
Remaining authorized amount for repurchase										$ 9,800,000