Provision for Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2016	[1]	Sep. 30, 2016	[1]	Jun. 30, 2016	[1]	Mar. 31, 2016	[1]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
U.S. federal income taxes (including foreign withholding taxes):
Current federal income taxes																	$ 154.1	$ 54.3	$ 117.8
Deferred federal income taxes																	(32.2)	36.6	46.9
Total federal income taxes																	121.9	90.9	164.7
State and local income taxes:
Current state and local income taxes																	18.8	8.2	15.1
Deferred state and local income taxes																	20.4	(1.5)	13.0
Total state and local income taxes																	39.2	6.7	28.1
Foreign income taxes:
Current foreign income taxes																	107.9	89.8	100.4
Deferred foreign income taxes																	12.9	10.6	(10.4)
Total foreign income taxes																	120.8	100.4	90.0
Total provision for income taxes	$ 166.1		$ 42.5		$ 75.4		$ (2.1)		$ 106.1		$ 63.8		$ 43.7		$ (15.6)		$ 281.9	$ 198.0	$ 282.8

[1] The three months ended September 30, 2017 included a tax benefit of $31.2 related to foreign tax credits from distributions of unremitted earnings, which was reversed during the three months ended December 31, 2017 as a result of the enactment of the Tax Cuts and Jobs Act. The three months ended December 31, 2017 included a net tax benefit of $36.0 as a result of the enactment of the Tax Cuts and Jobs Act. The three months ended March 31, 2016 included a tax benefit of $12.2 due to the reversal of valuation allowances. The three months ended June 30, 2016 included a tax benefit of $23.4 related to the conclusion and settlement of a tax examination of previous years. The three months ended December 31, 2016 included a tax benefit of $37.4 for refunds to be claimed on future amended U.S. federal returns.