Long-Term Debt (Narrative) (Details) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Senior Notes	$ 191,429,000	$ 227,143,000
Applicable margin interest on credit facility, low end	1.00%
Applicable margin interest on credit facility, high end	1.875%
Interest Rate Spread added to LIBOR Rate	1.00%
Applicable Margin Percentage, Interest Rate added to federal funds rate	0.50%
Line of Credit Facility, Amount Outstanding	34,680,000	10,000,000
Letter Of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	150,000,000
Line of Credit Facility, Amount Outstanding	57,800,000	52,400,000
Sweep Program [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	20,000,000
Five Year Senior Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Current borrowing capacity	200,000,000
Maximum borrowing capacity	300,000,000
Minimum increments under the additional borrowings	25,000,000
Interest Rate Spread added to LIBOR Rate	1.00%	1.125%
Senior Notes [Member]
Debt Instrument [Line Items]
Periodic principal payments	$ 35,700,000
Fixed interest rate, minimum	4.00%
Fixed interest rate, maximum	5.85%
Effective average interest rate	4.99%	5.07%
Maximum [Member]
Debt Instrument [Line Items]
Interest Rate Spread added to LIBOR Rate	1.125%
Minimum [Member]
Debt Instrument [Line Items]
Interest Rate Spread added to LIBOR Rate	1.00%