Debt (Narrative) (Details)	12 Months Ended			1 Months Ended			0 Months Ended									0 Months Ended	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Jan. 31, 2009 7.25% Unsecured Notes due February 1, 2019 [Member] USD ($)	Dec. 31, 2014 7.25% Unsecured Notes due February 1, 2019 [Member] USD ($)	Dec. 31, 2013 7.25% Unsecured Notes due February 1, 2019 [Member] USD ($)	Jun. 17, 2011 Four-Year Revolving Credit Agreement Dated, June 17, 2011 [Member] USD ($)	Dec. 31, 2014 USD $450 million portion of Revolver [Member] USD ($)	Dec. 31, 2013 USD $450 million portion of Revolver [Member] USD ($)	Dec. 20, 2012 USD $450 million portion of Revolver [Member] USD ($)	Jun. 17, 2011 USD $450 million portion of Revolver [Member] USD ($)	Dec. 31, 2014 AUD $50 million portion of Revolver [Member] USD ($)	Dec. 31, 2013 AUD $50 million portion of Revolver [Member] USD ($)	Sep. 07, 2011 AUD $50 million portion of Revolver [Member] AUD	Jun. 17, 2011 AUD $50 million portion of Revolver [Member] AUD	Sep. 07, 2011 Interest Rate Swap [Member] USD ($)	Dec. 31, 2014 Other Indebtedness [Member]	Dec. 31, 2013 Other Indebtedness [Member]
Debt Instrument [Line Items]
Borrowings outstanding								$ 0	$ 0	$ 240,000,000		$ 40,927,000	$ 44,625,000	50,000,000
Interest rate of debt instrument				7.25%	7.25%	7.25%				1.5107%
Minimum interest rate																	0.00%	0.00%
Maximum interest rate																	6.00%	6.00%
Debt Instrument, Maturity year, start																	Jan. 01, 2015	Jan. 01, 2014
Debt Instrument, Maturity year, end																	Dec. 31, 2017	Dec. 31, 2017
Debt Instrument, Maturity date				Feb. 01, 2019
Face amount of debt issued				400,000,000
Notes repurchase percentage of principal amount to be eligible for redemption				101.00%
Average borrowings outstanding	450,900,000	471,400,000
Weighted average interest rate of borrowings	7.00%	6.70%
Net interest expense incurred	34,500,000	33,800,000	32,600,000
Fair value of debt instrument					450,300,000	475,200,000
Current borrowing capacity											450,000,000				50,000,000
Maximum borrowing capacity							700,000,000
Minimum fee percentage for line of credit facility							0.08%
Maximum fee percentage for line of credit facility							0.20%
Line of credit facility, Expiration date							Jun. 17, 2015
Line of credit facility minimum requirement of shareholder's equity							1,500,000,000
Carrying value of debt instrument	398,308,000	397,893,000			398,300,000	397,900,000
Debt Instrument, Interest Rate Terms							Any borrowings are made on an unsecured basis and bear interest at (a) for U.S. dollar borrowings, at the Company’s option, either (i) a fluctuating interest rate equal to the highest of JPMorgan’s prime rate, 0.50 percent above the Federal funds rate or the one-month eurodollar rate plus 1%, or (ii) the eurodollar rate for the applicable interest period; or (b) for Australian dollar borrowings, the bank bill rate, in each case plus an applicable margin that depends on the Company’s long-term debt ratings.
Types Of Interest Rate Derivatives Used																interest rate swap
Notional Amount of Cash Flow Hedge Instruments																$ 50,000,000
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate																4.5275%