Consolidated Statement of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Director [Member]	Cumulative change related to the adoption of ASU 2016-13	Cumulative effect, period of adoption, adjusted balance	Common stock	Common stock Cumulative change related to the adoption of ASU 2016-13	Common stock Cumulative effect, period of adoption, adjusted balance	Retained earnings	Retained earnings Cumulative change related to the adoption of ASU 2016-13	Retained earnings Cumulative effect, period of adoption, adjusted balance	Accumulated other comprehensive income (loss)	Accumulated other comprehensive income (loss) Cumulative change related to the adoption of ASU 2016-13	Accumulated other comprehensive income (loss) Cumulative effect, period of adoption, adjusted balance
Equity, beginning balance at Dec. 31, 2019	$ 2,013.9		$ (24.1)	$ 1,989.8	$ 1,049.3	$ 0.0	$ 1,049.3	$ 953.6	$ (24.1)	$ 929.5	$ 11.0	$ 0.0	$ 11.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	29.3				0.0			29.3			0.0
Other comprehensive income (loss), net of tax expense	28.8				0.0			0.0			28.8
Common stock transactions:
Common shares purchased and retired	(32.6)				(32.6)			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	0.6				(0.6)			0.0			0.0
Stock options exercised, net of share tendered in payment of option price and income tax withholding amounts	1.4				1.4			0.0			0.0
Stock-based compensation expense	(61.2)				0.0			(61.2)			0.0
Equity, ending balance at Mar. 31, 2020	1,956.1				1,018.7			897.6			39.8
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	51.4				0.0			51.4			0.0
Other comprehensive income (loss), net of tax expense	(49.9)				0.0			0.0			(49.9)
Common stock transactions:
Common shares purchased and retired	(5.4)				(5.4)			0.0			0.0
Common shares issued (in shares)		604
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	2.4				2.4			0.0			0.0
Common cash dividend declared	(25.3)				0.0			(25.3)			0.0
Equity, ending balance at Mar. 31, 2021	$ 1,933.4				$ 938.5			$ 988.2			$ 6.7