Debt - Narrative (Details)	1 Months Ended				3 Months Ended	9 Months Ended	12 Months Ended
	Oct. 31, 2017 USD ($)	Oct. 31, 2016 USD ($)	Feb. 29, 2016 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) facility	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Long-term Debt Types [Abstract]
Short-term borrowings							$ 45,200,000	$ 162,800,000
Prepayment penalties							$ 8,300,000	0	$ 0
Senior Secured Credit Facility
Long-term Debt Types [Abstract]
Commitment Fee							0.25%
Letter of Credit
Long-term Debt Types [Abstract]
Maximum borrowing capacity							$ 40,000,000
Number of term loan facilities | facility							2
Undrawn letters of credit							$ 11,000,000
Letter of Credit | Two Unsecured Letter Of Credit Facilities
Long-term Debt Types [Abstract]
Amount available							94,000,000
Maximum borrowing amount							38,000,000
Undrawn letters of credit							56,000,000
Letter of Credit | Forty Million Unsecured Letter Of Credit Facility
Long-term Debt Types [Abstract]
Maximum borrowing amount							40,000,000
Letter of Credit | Fifty Four Million Unsecured Letter of Credit Facility
Long-term Debt Types [Abstract]
Maximum borrowing amount							$ 54,000,000
Minimum | Senior Secured Credit Facility
Long-term Debt Types [Abstract]
Commitment Fee							0.15%
Maximum | Senior Secured Credit Facility
Long-term Debt Types [Abstract]
Commitment Fee							0.40%
Revolving Credit Facility
Long-term Debt Types [Abstract]
Maximum borrowing capacity							$ 525,000,000
Revolving Credit Facility | Senior Secured Credit Facility
Long-term Debt Types [Abstract]
Maximum borrowing capacity	$ 1,000,000,000
Line of credit maturity period	5 years
Maximum borrowing amount							1,000,000,000
Term Loan A | Senior Secured Credit Facility
Long-term Debt Types [Abstract]
Long-term Debt, Gross	$ 500,000,000
Annual amortization percent	5.00%
Debt maturity period	5 years
Senior unsecured notes | Six hundred million senior unsecured notes
Long-term Debt Types [Abstract]
Long-term Debt, Gross	$ 600,000,000
Debt maturity period	10 years
Private Placement Notes
Long-term Debt Types [Abstract]
Long-term Debt, Gross							0	85,600,000
Repayments of unsecured debt						$ 86,000,000
Prepayment penalties					$ 6,500,000	7,000,000
Term Loan
Long-term Debt Types [Abstract]
Long-term Debt, Gross							0	65,600,000
Repayments of unsecured debt	$ 64,000,000
Prepayment penalties	$ 2,000,000
Term loan				$ 75,000,000
Multi-currency revolving facility
Long-term Debt Types [Abstract]
Maximum borrowing capacity							10,000,000
Multi-currency revolving facility | Letter of Credit
Long-term Debt Types [Abstract]
Undrawn letters of credit							$ 5,000,000
LIBOR | Senior Secured Credit Facility
Long-term Debt Types [Abstract]
Interest rate margin							1.75%
LIBOR | Senior Secured Credit Facility | Minimum
Long-term Debt Types [Abstract]
Interest rate margin							1.25%
LIBOR | Senior Secured Credit Facility | Maximum
Long-term Debt Types [Abstract]
Interest rate margin							2.50%
LIBOR | Term Loan
Long-term Debt Types [Abstract]
Interest rate margin				1.75%
Base Rate | Senior Secured Credit Facility
Long-term Debt Types [Abstract]
Interest rate margin							0.75%
Base Rate | Senior Secured Credit Facility | Minimum
Long-term Debt Types [Abstract]
Interest rate margin							0.25%
Base Rate | Senior Secured Credit Facility | Maximum
Long-term Debt Types [Abstract]
Interest rate margin							1.50%
Uncommitted Credit Facilities
Long-term Debt Types [Abstract]
Short-term borrowings							$ 0	$ 108,300,000
Uncommitted Credit Facilities | Uncommitted Facility Agreement
Long-term Debt Types [Abstract]
Maximum borrowing capacity		$ 100,000,000
Maximum maturity period		30 days
Uncommitted Credit Facilities | Twenty Four Million Uncommitted Credit Facility
Long-term Debt Types [Abstract]
Amount available			$ 24,000,000
Uncommitted Credit Facilities | LIBOR | Uncommitted Facility Agreement
Long-term Debt Types [Abstract]
Interest rate margin		1.00%
Uncommitted Credit Facilities | LIBOR | Twenty Four Million Uncommitted Credit Facility
Long-term Debt Types [Abstract]
Interest rate margin			1.00%
Unsecured term loan facility | One Hundred Million Uncommitted Credit Facility
Long-term Debt Types [Abstract]
Maximum borrowing capacity					$ 100,000,000	$ 100,000,000
Short-term borrowings							$ 85,000,000
Unsecured term loan facility | Minimum | One Hundred Million Uncommitted Credit Facility
Long-term Debt Types [Abstract]
Interest rate margin							1.00%
Unsecured term loan facility | Maximum | One Hundred Million Uncommitted Credit Facility
Long-term Debt Types [Abstract]
Interest rate margin							1.875%