Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Adjustments for New Accounting Principle, Early Adoption [Member]	Restricted Stock [Member]
Beginning Balance at Dec. 31, 2016	$ 982.6	$ 296.1	$ 694.7	$ (8.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	106.5	0.0	106.5	0.0
Other comprehensive loss, net of tax	(3.8)	$ 0.0	0.0	(3.8)
Stock Repurchased and Retired During Period, Shares		33,063
Common stock transactions:
Common shares purchased and retired	(1.3)	$ (1.3)	0.0	0.0
Stock Issued During Period, Value, New Issues	386.0	386.0	0.0	0.0
Non-vested common shares forfeited or canceled	0.0		0.0
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	2.4	2.4	0.0	0.0
Stock compensation expense	3.8	3.8	0.0	0.0		$ 3.3
Stock-based compensation expense	(48.6)	0.0	(48.6)	0.0
Cash dividends declared:
Common (per share)	(1,427.6)	(687.0)	(752.6)	(12.0)
Common Stock, Value, Issued	0.0	0.0	0.0	0.0
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	0.0		3.1	(3.1)
Net income	160.2
Other comprehensive loss, net of tax	(9.5)	$ 0.0	0.0	(9.5)
Stock Repurchased and Retired During Period, Shares		24,271
Common shares purchased and retired	(1.0)	$ (1.0)	0.0	0.0
Stock Issued During Period, Value, New Issues	173.3	173.3	0.0	0.0
Non-vested common shares forfeited or canceled	0.0	0.0	0.0	0.0
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	1.8
Stock compensation expense	5.6	5.6	0.0	0.0		5.6
Stock-based compensation expense	(64.1)		(64.1)	0.0		$ 0.0
Common (per share)	(1,693.9)	(866.7)	(851.8)	(24.6)
Ending Balance at Dec. 31, 2018	1,693.9			(24.6)
Accumulated Other Comprehensive Income (Loss), Net of Tax	(24.6)
Common stock transactions:
Common Stock, Value, Issued	0.0	0.0	0.0	0.0
Net income	181.0	0.0	181.0	0.0
Other comprehensive loss, net of tax	35.6	$ 0.0		35.6
Other comprehensive loss, net of tax | Accounting Standards Update 2018-02 [Member]					$ 0.0
Stock Repurchased and Retired During Period, Shares		43,560
Common shares purchased and retired	(2.5)	$ (2.5)	0.0	0.0
Stock Issued During Period, Value, New Issues	176.1	176.1	0.0	0.0
Non-vested common shares issued	0.0
Non-vested common shares forfeited or canceled	0.0			0.0
Stock Issued, Value, Stock Options Exercised, Net of Tax Benefit (Expense)	1.0	1.0	0.0	0.0
Stock compensation expense	8.0	8.0	0.0	0.0
Cash dividends declared:
Common (per share)	(79.2)	0.0	(79.2)	0.0
Ending Balance at Dec. 31, 2019	2,013.9	1,049.3	181.0	11.0
Accumulated Other Comprehensive Income (Loss), Net of Tax	11.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	2,013.9		$ 953.6
Common stock transactions:
Common Stock, Value, Issued	$ 0.0	$ 0.0		$ 0.0