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Common Stock (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 45 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Jun. 30, 2014 | Jun. 30, 2015 | Dec. 31, 2014 | Oct. 23, 2014 | Dec. 05, 2013 | Sep. 26, 2011
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |
Common stock, outstanding | 62,784,449 |  | 62,784,449 |  | 62,784,449 | 62,981,229 |  |  |
Common stock, issued | 102,562 | 112,108 | 281,490 | 562,724 |  |  |  |  |
Number of shares repurchased |  |  | 500,000 |  | 8,000,000 |  |  |  |
Stock repurchased (in US dollars) |  |  | $ 21,148,000 | $ 18,724,000 | $ 304,700,000 |  |  |  |
RepurchaseProgramSep262011 |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  |  | $ 100,000,000
RepurchaseProgramDec052013 |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  | $ 100,000,000 |
RepurchaseProgramOct232014 |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  | $ 250,000,000 |  |
Remaining authorized repurchase amount (in US dollars) | $ 145,300,000 |  | $ 145,300,000 |  | $ 145,300,000 |  |  |  |

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