Borrowings - Senior Notes (Details) - USD ($)	1 Months Ended		12 Months Ended
	Mar. 31, 2013	Jan. 31, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2014
Debt Instrument [Line Items]
Borrowing Outstanding			$ 1,575,500,000	$ 1,267,600,000
Interest			65,500,000	61,300,000	$ 58,000,000
3.875% Senior Notes Due 2/01/2023
Debt Instrument [Line Items]
Interest rate on senior notes (as a percent)		3.875%
Borrowing Outstanding		$ 500,000,000.0
Senior notes percentage of par value		99.966%
Senior notes redemption terms percentage to principal amount		100.00%
Interest			19,800,000	19,800,000	19,800,000
Fair value			520,400,000	512,800,000
3.875% Senior Notes Due 2/01/2023 | Treasury Rate
Debt Instrument [Line Items]
Margin range of interest rates (as a percent)		0.30%
5.625% Senior Notes Due 3/30/2043
Debt Instrument [Line Items]
Interest rate on senior notes (as a percent)	5.625%					5.625%
Borrowing Outstanding	$ 400,000,000.0					$ 200,000,000.0
Senior notes percentage of par value	99.583%					104.315%
Senior notes redemption terms percentage to principal amount	100.00%
Interest			33,700,000	33,700,000	$ 33,700,000
Fair value			$ 696,300,000	$ 603,100,000
5.625% Senior Notes Due 3/30/2043 | Treasury Rate
Debt Instrument [Line Items]
Margin range of interest rates (as a percent)	0.40%