Indebtedness (Details) (USD $)			0 Months Ended	12 Months Ended		12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Jun. 28, 2013 Credit Facility	Dec. 31, 2013 Credit Facility	Dec. 31, 2012 Credit Facility	Dec. 31, 2013 Senior Notes item tranche	Dec. 31, 2013 Senior Notes Maximum	Dec. 31, 2013 Senior Notes Minimum	Dec. 31, 2013 Senior 5.0% unsecured notes due, 2018 ("Series A Notes")	Dec. 31, 2012 Senior 5.0% unsecured notes due, 2018 ("Series A Notes")	Dec. 31, 2013 Senior 5.75% unsecured notes due, 2021 ("Series B Notes")	Dec. 31, 2012 Senior 5.75% unsecured notes due, 2021 ("Series B Notes")	Jan. 31, 2011 Senior 5.60% unsecured notes due, 2011
Indebtedness
Fair value of outstanding indebtedness	$ 203,700,000
Indebtedness
Interest rate of debt (as a percent)									5.00%		5.75%		5.60%
Consolidated leverage ratio						0.4	3.0
Consolidated interest coverage ratio						42.2		4.0
Number of consecutive quarters for which the maximum consolidated leverage ratio is required to be maintained under financial covenants						4
Number of consecutive quarters for which the minimum consolidated interest coverage ratio is required to be maintained under financial covenants						4
Number of tranches of debt issuance						2
Revolving credit facility term			5 years
Maximum borrowing capacity			125,000,000
Variable Rate Basis				LIBOR
Amount that borrowing capacity can be expanded upon entity's request			200,000,000
Borrowing outstanding under the facility				0	0
fair value of outstanding indebtedness	203,700,000
Summary of long-term debt
Total long-term debt	$ 190,000,000	$ 190,000,000							$ 95,000,000	$ 95,000,000	$ 95,000,000	$ 95,000,000