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Common Stock (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 81 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Dec. 31, 2017 | Oct. 25, 2017 | Oct. 28, 2015 | Oct. 23, 2014 | Dec. 05, 2013 | Sep. 26, 2011
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Common stock, outstanding | 58,173,108 |  | 58,173,108 |  |  | 58,173,108 | 58,798,428 |  |  |  |  |
Common stock, issued | 72,777 | 76,347 | 170,930 |  | 193,381 |  |  |  |  |  |  |
Stock repurchased (in US dollars) |  |  | $ 60,508,000 | [1] | $ 63,430,000 |  |  |  |  |  |  |
Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of shares repurchased |  |  |  |  |  | 13,600,000 |  |  |  |  |  |
Stock repurchased (in US dollars) |  |  |  |  | $ 10,000 | $ 635,800,000 |  |  |  |  |  |
Remaining authorized repurchase amount (in US dollars) | $ 114,200,000 |  | $ 114,200,000 |  |  | $ 114,200,000 |  |  |  |  |  |
Common Stock [Member] | RepurchaseProgramSep262011 |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  |  |  |  |  | $ 100,000,000
Common Stock [Member] | RepurchaseProgramDec052013 |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  |  |  |  | $ 100,000,000 |
Common Stock [Member] | RepurchaseProgramOct232014 |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  |  |  | $ 250,000,000 |  |
Common Stock [Member] | RepurchaseProgramOct282015 |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  |  | $ 200,000,000 |  |  |
Number of shares repurchased |  |  | 796,250 |  |  |  |  |  |  |  |  |
Stock repurchased (in US dollars) |  |  | $ 60,500,000 |  |  |  |  |  |  |  |  |
Common Stock [Member] | RepurchaseProgramOct252017 |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  | $ 100,000,000 |  |  |  |
[1] | Beginning June 1, 2017, shares of common stock repurchased are held as treasury stock by the Company.

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- Definition Number of shares that have been repurchased during the period and have not been retired and are not held in treasury. Some state laws may govern the circumstances under which an entity may acquire its own stock and prescribe the accounting treatment therefore. This element is used when state law does not recognize treasury stock. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(28)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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