Condensed Consolidated Balance Sheets (unaudited) (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Assets
Accumulated depreciation	$ 199,767	$ 185,994
Equity
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	10,000,000us-gaap_PreferredStockSharesAuthorized	10,000,000us-gaap_PreferredStockSharesAuthorized
Preferred stock, shares issued (in shares)	5,750,000us-gaap_PreferredStockSharesIssued	5,750,000us-gaap_PreferredStockSharesIssued
Preferred stock, shares outstanding (in shares)	5,750,000us-gaap_PreferredStockSharesOutstanding	5,750,000us-gaap_PreferredStockSharesOutstanding
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	125,000,000us-gaap_CommonStockSharesAuthorized	125,000,000us-gaap_CommonStockSharesAuthorized
Common stock, shares issued (in shares)	59,234,056us-gaap_CommonStockSharesIssued	59,047,001us-gaap_CommonStockSharesIssued
Common stock, shares outstanding (in shares)	59,234,056us-gaap_CommonStockSharesOutstanding	59,047,001us-gaap_CommonStockSharesOutstanding

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