Debt Credit Facility Additional Information (Details) (USD $)			3 Months Ended
	Mar. 31, 2013	Aug. 05, 2011	Mar. 31, 2013 U.S. Domestic Subsidiaries [Member]	Mar. 31, 2013 First-Tier Foreign Subsidiaries [Member] Non-voting Stock [Member]	Mar. 31, 2013 Scheduled Quarterly Repayments through June 2013 [Domain]	Mar. 31, 2013 Scheduled Quarterly Repayments September 2013 through June 2014 [Domain]	Mar. 31, 2013 Scheduled Quarterly Repayments September 2014 through June 2016 [Domain]	Mar. 31, 2013 Minimum [Member]	Mar. 31, 2013 Maximum [Member]	Mar. 31, 2013 Maximum [Member] First-Tier Foreign Subsidiaries [Member] Voting Stock [Member]	Mar. 31, 2013 Alternate base rate (1) [Member]	Mar. 31, 2013 Alternate base rate (2) [Member]	Mar. 31, 2013 Alternate base rate (3) [Member]	Mar. 31, 2013 LIBOR interest rate [Member]	Apr. 02, 2012 Increased Line of Credit Borrowing Capacity [Member]	Aug. 05, 2011 Line of Credit Borrowing Capacity at Inception [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 300,000,000
Line of credit facility, maximum borrowing capacity															660,000,000	500,000,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage								0.20%	0.40%
Pledge of Capital Stock			100.00%	100.00%						66.00%
Repayments of Secured Debt					3,800,000	5,600,000	7,500,000
Debt Instrument, Description of Variable Rate Basis											the prime rate	the Federal Reserve effective rate	one month LIBOR	the LIBOR rate
Debt Instrument, Basis Spread on Variable Rate												0.50%	1.00%	1.25%
Applicable Interest Rate														1.46%
Line of credit facility, amount outstanding	125,000,000
Letters of Credit Outstanding, Amount	47,800,000
Line of credit facility, remaining borrowing capacity	$ 487,200,000