4. Debt Maturity Summary (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013		Dec. 31, 2012
Debt Instrument [Line Items]
DateOfAnnualPrincipalPayments	December 15
Long-term Debt	$ 725,000		$ 707,600
Total Debt in Percentage	100.00%
Debt, Weighted Average Interest Rate	6.90%
2013 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Percentage of Total Debt	0.00%
Debt, Weighted Average Interest Rate	0.00%
2014 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Percentage of Total Debt	0.00%
Debt, Weighted Average Interest Rate	0.00%
2015 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	125,000
Percentage of Total Debt	17.20%
Debt, Weighted Average Interest Rate	8.50%
2016 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	188,750
Percentage of Total Debt	26.00%
Debt, Weighted Average Interest Rate	6.30%
2017 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	308,750
Percentage of Total Debt	42.70%
Debt, Weighted Average Interest Rate	8.30%
2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	102,500
Percentage of Total Debt	14.10%
Debt, Weighted Average Interest Rate	2.00%
Fixed Rate Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Amount	550,000		550,000
Percentage of Total Debt	76.00%
Fixed Rate Debt [Member] | 2013 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Fixed Rate Debt [Member] | 2014 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Fixed Rate Debt [Member] | 2015 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	125,000	[1]
Fixed Rate Debt [Member] | 2016 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	125,000	[1]
Fixed Rate Debt [Member] | 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	300,000	[1]
Fixed Rate Debt [Member] | 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Floating Rate Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Amount	175,000		157,600
Percentage of Total Debt	24.00%
Floating Rate Debt [Member] | 2013 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Floating Rate Debt [Member] | 2014 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Floating Rate Debt [Member] | 2015 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Floating Rate Debt [Member] | 2016 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	63,750	[2],[3]
Floating Rate Debt [Member] | 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	8,750	[2]
Floating Rate Debt [Member] | 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	102,500	[2]
Unsecured Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Mar. 21, 2016
AccThreeTermLoan [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Mar. 27, 2018
Debt Instrument, Frequency of Periodic Payment	Quarterly
AccThreeTermLoan [Member] | BeginningAprilOneTwoThousandSixteen [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	1,250
Debt Instrument, Date of First Required Payment	Apr. 01, 2016
AccThreeTermLoan [Member] | BeginningAprilOneTwoThousandSeventeen [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 2,500
Debt Instrument, Date of Increased Required Payment	Apr. 01, 2017

[1] The Unsecured Notes have mandatory amortization payments due December 15 of each respective year.

[2] The ACC3 Term Loan matures on March 27, 2018 with no extension option. Quarterly principal payments of $1.25 million begin on April 1, 2016, increase to $2.5 million on April 1, 2017 and continue through maturity.

[3] The Unsecured Credit Facility matures on March 21, 2016 with a one-year extension option.