Long-Term Debt (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Senior Notes	$ 227,143,000	$ 262,857,000
Maximum borrowing capacity	300,000,000
Minimum increments under the additional borrowings	25,000,000
Interest Rate Spread added to LIBOR Rate	1.00%
Interest Rate Spread added to Federal Funds Rate	0.50%
Line of credit, outstanding	10,000,000	0
Facing fee on outstanding amount of letter of credit	0.125%
Minimum amount of dividends payable to shareholder as per credit agreement	20.0
Percentage of preceding year net income payable as dividend	25.00%
Collateralized property and equipment book value	7,600,000
Letter Of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity	150,000,000
Line of credit, outstanding	52,400,000	49,900,000
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
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	1.125%	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	5.07%	5.17%
Maximum [Member]
Debt Instrument [Line Items]
Interest Rate Spread added to LIBOR Rate	1.125%
Letter of credit fee	1.125%
Commitment fee percentage	0.20%
Line Of Credit Facility Commitment Stated Fee Percentage	0.30%
Minimum [Member]
Debt Instrument [Line Items]
Interest Rate Spread added to LIBOR Rate	1.00%
Letter of credit fee	1.00%
Commitment fee percentage	0.175%
Line Of Credit Facility Commitment Stated Fee Percentage	0.175%