Short-Term Borrowings (Details) € in Millions	12 Months Ended		14 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 EUR (€)	Jan. 24, 2017	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Short-term Debt [Line Items]
Short-term borrowings	$ 436,215,000			$ 525,842,000
Debt interest rate	2.51%			1.77%
Floating rate puttable notes
Short-term Debt [Line Items]
Short-term borrowings	$ 108,240,000			$ 96,455,000
Debt issued during period | €		€ 91.0
Equity-linked notes
Short-term Debt [Line Items]
Short-term borrowings	23,324,000			0
Equity-linked notes matured in July 2017
Short-term Debt [Line Items]
Debt issued during period	30,600,000
Equity-linked notes matured in September 2017
Short-term Debt [Line Items]
Debt issued during period	4,200,000
Bank loans
Short-term Debt [Line Items]
Short-term borrowings	304,651,000			372,301,000
Credit Facility | Intraday Credit Facility
Short-term Debt [Line Items]
Committed amount	$ 150,000,000.0
Credit Facility | Intraday Credit Facility | Federal Funds Rate
Short-term Debt [Line Items]
Basis spread on variable rate	0.50%
Credit Facility | Secured Revolving Loan Facility
Short-term Debt [Line Items]
Committed amount					$ 50,000,000.0
Secured revolving loan facilities | Secured Revolving Loan Facility
Short-term Debt [Line Items]
Short-term borrowings	$ 0			$ 57,086,000
Secured revolving loan facilities | Secured Revolving Loan Facility | LIBOR
Short-term Debt [Line Items]
Basis spread on variable rate			4.25%