Financial Instruments - Derivatives and Hedging (Details) € in Millions, $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 EUR (€)	Dec. 31, 2017 USD ($)
Foreign Exchange Forward
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	$ 35.1	$ 35.1		$ 31.8
Derivative remaining maturity		1 year
Embedded Derivative Financial Instruments
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	11.3	$ 11.3		9.7
Derivative assets (liabilities), at fair value	0.3	0.3		(0.4)
Interest rate swaps | Notes Payable to Banks | USD Notes
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	$ 1,130.0	$ 1,130.0
Interest rate swap rate (as a percent)	1.96%	1.96%	1.96%
Interest rate swaps | Notes Payable to Banks | Euro Notes
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount | €			€ 277
Interest rate swap rate (as a percent)	1.20%	1.20%	1.20%
Not Designated as Hedging Instrument | Foreign Exchange Forward
Derivative Instruments, Gain (Loss) [Line Items]
Derivative notional amount	$ 657.0	$ 657.0		$ 615.0
Derivative remaining maturity		1 year
Designated as Hedging Instrument | Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Amount of ineffectiveness on net investment hedges	0.2	$ 0.6
Gain (loss) to be reclassified during next 12 months	$ 4.2	$ 4.2