Long-Term Debt (Narrative) (Details) (USD $)	3 Months Ended		3 Months Ended				3 Months Ended								3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Series A [Member] Private Placement [Member]	Mar. 31, 2012 Series B [Member] Private Placement [Member]	Mar. 31, 2012 Master Agreement [Member] Y	Dec. 31, 2011 Master Agreement [Member]	Mar. 31, 2012 Master Agreement [Member] Series C [Member]	Mar. 31, 2012 Master Agreement [Member] Series D [Member]	Mar. 31, 2012 Master Agreement [Member] Series E [Member]	Mar. 31, 2012 Line Of Credit [Member]	Mar. 31, 2012 LIBOR [Member]	Mar. 31, 2012 Repayment Of Debt [Member] Private Placement [Member]	Dec. 31, 2011 Repayment Of Debt [Member] Private Placement [Member]	Jul. 31, 2004 Senior Notes [Member] Private Placement [Member]	Mar. 31, 2012 Sun Trust Term Loan [Member]	Mar. 31, 2012 Sun Trust Revolver [Member]	Mar. 31, 2012 JPMorgan Chase Bank, N.A. [Member]	Mar. 31, 2012 JPM Bridge Facility [Member]	Mar. 31, 2012 JPM Term Loan [Member]
Debt Instrument [Line Items]
Maximum shelf facility capacity					$ 200,000,000
Notes redeemed			100,000,000
Unsecured senior notes outstanding	450,000,000	250,000,000	100,000,000	100,000,000	150,000,000	150,000,000	25,000,000	25,000,000	100,000,000			100,000,000	100,000,000	200,000,000
Length of the issuance period of long-term debt (in years)					3
Long-term debt, maturities, repayment terms, maximum number of years					ten
Debt instrument, issuance date			September 15, 2004	July 15, 2004			December 22, 2006	February 1, 2008	September 15, 2011
Debt instrument, maturity date				Jul. 15, 2014			Dec. 22, 2016	Jan. 15, 2015	Sep. 15, 2018							Dec. 31, 2016		Feb. 03, 2012
Debt instrument interest rate stated percentage			5.57%	6.08%			5.66%	5.37%	4.50%
Lending commitment amount										50,000,000
Revolving credit facility, maturity date	December 20, 2011
Revolving credit facility, extended maturity date	June 3, 2013
Potential maximum increase in lending commitment amount										100,000,000
Revolving credit facility in the maximum principal amount										75,000,000					150,000,000	100,000,000
Increase in revolving credit facility															50,000,000	50,000,000
Reduction in maximum principal amount										20,000,000
Revolving and term loan															$ 100,000,000	$ 50,000,000	$ 50,000,000	$ 50,000,000	$ 100,000,000
Availability fees, minimum										0.175%
Availability fees, maximum										0.25%
Letter of credit usage fee, minimum										1.00%
Letter of credit usage fee, maximum										1.40%
30-day LIBOR Rate											0.24125%
30-day Adjusted LIBOR Rate											0.25%
LIBOR below base rate										1.00%
Percentage of LIBOR advances										1.00%
Availability fees, percentage										0.175%
Adjusted LIBOR Rate																			1.00%
Interest rate, minimum										1.00%
Interest rate, maximum										1.40%