Long-Term Debt (Narrative) (Details)	1 Months Ended	12 Months Ended
	Mar. 31, 2015 USD ($)	Dec. 31, 2015 USD ($) facility	Jun. 30, 2015 USD ($)	Dec. 31, 2014 USD ($)
Long-term Debt Types [Abstract]
Facility fee percent		0.20%
Long-term Debt		$ 401,400,000		$ 407,200,000
Amended Revolving Facility
Long-term Debt Types [Abstract]
Maximum borrowing amount	$ 480,000,000
Amount available		362,000,000
Unsecured Multi-currency Debt Facilities
Long-term Debt Types [Abstract]
Maximum borrowing amount		33,000,000
Amount available		40,000,000
Undrawn letters of credit		7,300,000
Unsecured Bilateral Commited Credit Facilities
Long-term Debt Types [Abstract]
Maximum borrowing amount		20,000,000
Amount available		24,500,000.0
Unsecured Letter of Credit Facilities
Long-term Debt Types [Abstract]
Maximum borrowing amount		23,000,000
Amount available		94,000,000
Undrawn letters of credit		$ 70,700,000
Revolving Credit Facility
Long-term Debt Types [Abstract]
Maximum borrowing capacity	525,000,000
Revolving Credit Facility | Unsecured Multi-currency Debt Facilities
Long-term Debt Types [Abstract]
Number of term loan facilities | facility		2
Letter of Credit
Long-term Debt Types [Abstract]
Maximum borrowing capacity			$ 40,000,000
Unsecured notes outstanding		$ 5,000,000
Letter of Credit | Unsecured Letter of Credit Facilities
Long-term Debt Types [Abstract]
Number of term loan facilities | facility		2
December 2015 Multi-currency Facility | Unsecured Multi-currency Debt Facilities
Long-term Debt Types [Abstract]
Maximum borrowing amount		$ 20,000,000
February 2017 Multi-currency Facility
Long-term Debt Types [Abstract]
Maximum borrowing amount		20,000,000
Eighty Five Million Unsecured Letter of Credit Facility | Unsecured Letter of Credit Facilities
Long-term Debt Types [Abstract]
Maximum borrowing amount		54,000,000
Twenty Four Million Unsecured Letter of Credit Facility | Unsecured Letter of Credit Facilities
Long-term Debt Types [Abstract]
Maximum borrowing amount		$ 40,000,000
Minimum | Amended Revolving Facility
Long-term Debt Types [Abstract]
Facility fee percent		0.125%
Minimum | Unsecured Multi-currency Debt Facilities
Long-term Debt Types [Abstract]
Price range		0.90%
Minimum | Unsecured Bilateral Commited Credit Facilities
Long-term Debt Types [Abstract]
Price range		1.20%
Maximum | Amended Revolving Facility
Long-term Debt Types [Abstract]
Facility fee percent		0.30%
Maximum | Unsecured Multi-currency Debt Facilities
Long-term Debt Types [Abstract]
Price range		2.00%
Maximum | Unsecured Bilateral Commited Credit Facilities
Long-term Debt Types [Abstract]
Price range		1.575%
Unsecured notes issued in a private placement | Private Placement Notes
Long-term Debt Types [Abstract]
Principal		$ 93,000,000
Unsecured notes issued in a private placement | Term Loan
Long-term Debt Types [Abstract]
Debt Instrument, Face Amount	75,000,000	70,900,000		$ 0
Debt Instrument, Unused Borrowing Capacity, Amount	$ 33,700,000
Long-term Debt		71,000,000
Unsecured notes issued in a private placement | Private Placement Notes Series A | Private Placement Notes
Long-term Debt Types [Abstract]
Unsecured notes outstanding		$ 43,000,000
Fixed interest rate		4.57%
Unsecured notes issued in a private placement | Private Placement Notes Series B | Private Placement Notes
Long-term Debt Types [Abstract]
Unsecured notes outstanding		$ 50,000,000
Fixed interest rate		5.20%
LIBOR | Amended Revolving Facility
Long-term Debt Types [Abstract]
Price range		1.30%
LIBOR | Minimum | Amended Revolving Facility
Long-term Debt Types [Abstract]
Price range		1.00%
LIBOR | Maximum | Amended Revolving Facility
Long-term Debt Types [Abstract]
Price range		1.70%
LIBOR | Unsecured notes issued in a private placement | Term Loan
Long-term Debt Types [Abstract]
Price range	1.75%
Base Rate | Minimum | Amended Revolving Facility
Long-term Debt Types [Abstract]
Price range		0.00%
Base Rate | Maximum | Amended Revolving Facility
Long-term Debt Types [Abstract]
Price range		0.70%