Consolidated Statement of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Common stock	Retained earnings	Accumulated other comprehensive income (loss)
Equity, beginning balance at Dec. 31, 2020	$ 1,959.8	$ 941.1	$ 962.1	$ 56.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	93.9	0.0	93.9	0.0
Other comprehensive income (loss), net of tax expense	(32.5)	0.0	0.0	(32.5)
Common stock transactions:
Common shares purchased and retired	(5.4)	(5.4)	0.0	0.0
Common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0	0.0	0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	0.4	0.4	0.0	0.0
Stock-based compensation expense	5.5	5.5	0.0	0.0
Common cash dividend declared	(50.8)	0.0	(50.8)	0.0
Equity, ending balance at Jun. 30, 2021	1,970.9	941.6	1,005.2	24.1
Equity, beginning balance at Mar. 31, 2021	1,933.4	938.5	988.2	6.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	42.5	0.0	42.5	0.0
Other comprehensive income (loss), net of tax expense	17.4	0.0	0.0	17.4
Common stock transactions:
Common shares purchased and retired	0.0	0.0	0.0	0.0
Common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares issued	0.0	0.0	0.0	0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	3.1	3.1	0.0	0.0
Stock-based compensation expense	(25.5)	0.0	(25.5)	0.0
Equity, ending balance at Jun. 30, 2021	1,970.9	941.6	1,005.2	24.1
Equity, beginning balance at Dec. 31, 2021	1,986.6	945.0	1,052.6	(11.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	30.7	0.0	30.7	0.0
Other comprehensive income (loss), net of tax expense	(318.8)	0.0	0.0	(318.8)
Common stock transactions:
Common shares purchased and retired	(65.9)	(65.9)	0.0	0.0
Common shares issued	1,722.5	1,722.5	0.0	0.0
Non-vested common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0	0.0	0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	0.1	0.1	0.0	0.0
Stock-based compensation expense	6.2	6.2	0.0	0.0
Common cash dividend declared	(89.5)	0.0	(89.5)	0.0
Equity, ending balance at Jun. 30, 2022	3,271.9	2,607.9	993.8	(329.8)
Equity, beginning balance at Mar. 31, 2022	3,441.1	2,668.6	974.5	(202.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	64.1	0.0	64.1	0.0
Other comprehensive income (loss), net of tax expense	(127.8)	0.0	0.0	(127.8)
Common stock transactions:
Common shares purchased and retired	(64.5)	(64.5)	0.0	0.0
Common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares issued	0.0	0.0	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0	0.0	0.0
Stock-based compensation expense	3.8	3.8	0.0	0.0
Common cash dividend declared	(44.8)	0.0	(44.8)	0.0
Equity, ending balance at Jun. 30, 2022	$ 3,271.9	$ 2,607.9	$ 993.8	$ (329.8)