Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, assets	$ 4,565	$ 9,333
Investments held in Rabbi Trusts	38,681	32,519
Fair Value, Measurements, Recurring [Member]
Assets and liabilities measured at fair value on a recurring basis
Total assets at fair value	43,723	41,852
Total liabilities at fair value	212	1,162
Fair Value, Measurements, Recurring [Member] | Cash and Cash Equivalents [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	4,418	7,101
Fair Value, Measurements, Recurring [Member] | U.S. Equity Mutual Funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	23,589	16,479
Fair Value, Measurements, Recurring [Member] | Foreign equity mutual funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	4,724	4,323
Fair Value, Measurements, Recurring [Member] | Fixed income mutual funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	5,950	4,616
Fair Value, Measurements, Recurring [Member] | Prepaid Expenses and Other Current Assets [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, assets	477
Fair Value, Measurements, Recurring [Member] | Direct Financing Leases and Other Assets [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, assets	4,565	9,333
Investments held in Rabbi Trusts	38,681	32,519
Fair Value, Measurements, Recurring [Member] | Other non-current liabilities [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, liabilities	212	1,162
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Assets and liabilities measured at fair value on a recurring basis
Total assets at fair value	38,681	32,519
Interest rate swaps, liabilities		0
Total liabilities at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Cash and Cash Equivalents [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	4,418	7,101
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Equity Mutual Funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	23,589	16,479
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Foreign equity mutual funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	4,724	4,323
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Fixed income mutual funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	5,950	4,616
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Prepaid Expenses and Other Current Assets [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, assets	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Direct Financing Leases and Other Assets [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, assets	0	0
Investments held in Rabbi Trusts	38,681	32,519
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Other non-current liabilities [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, liabilities	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Assets and liabilities measured at fair value on a recurring basis
Total assets at fair value	5,042	9,333
Interest rate swaps, liabilities		1,162
Total liabilities at fair value	212	1,162
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Cash and Cash Equivalents [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Equity Mutual Funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Foreign equity mutual funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Fixed income mutual funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Prepaid Expenses and Other Current Assets [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, assets	477
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Direct Financing Leases and Other Assets [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, assets	4,565	9,333
Investments held in Rabbi Trusts	0	0
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Other non-current liabilities [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, liabilities	212
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Assets and liabilities measured at fair value on a recurring basis
Total assets at fair value	0	0
Interest rate swaps, liabilities		0
Total liabilities at fair value	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Cash and Cash Equivalents [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Equity Mutual Funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Foreign equity mutual funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Fixed income mutual funds [Member]
Assets and liabilities measured at fair value on a recurring basis
Investments held in Rabbi Trusts	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Prepaid Expenses and Other Current Assets [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, assets	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Direct Financing Leases and Other Assets [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, assets	0	0
Investments held in Rabbi Trusts	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Other non-current liabilities [Member]
Assets and liabilities measured at fair value on a recurring basis
Interest rate swaps, liabilities	$ 0