Long-Term Debt (Narrative) (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Senior Notes	$ 155,714,000	$ 191,429,000
Line of Credit Facility, Amount Outstanding	28,203,000	0
Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Amount Outstanding	63,300,000	57,700,000
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	4.87%	4.99%
Sweep Program [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases	20,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Current borrowing capacity	200,000,000
Line Of Credit, Additional Capacity	100,000,000
Minimum increments under the additional borrowings	25,000,000
Interest Rate Spread added to Rate	1.00%	1.00%
Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Interest Rate Spread added to Rate	1.875%
Revolving Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest Rate Spread added to Rate	1.00%
Letter of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Capacity Available for Specific Purpose Other than for Trade Purchases	$ 150,000,000
Federal Funds Effective Swap Rate [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest Rate Spread added to Rate	0.50%
London Interbank Offered Rate (LIBOR) [Member] | Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Interest Rate Spread added to Rate	1.00%