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Common Stock (Details) - USD ($) | 3 Months Ended | 6 Months Ended | 93 Months Ended |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Dec. 31, 2018 | Oct. 23, 2018 | 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 | 56,129,224 |  | 56,129,224 |  | 56,129,224 | 55,983,763 |  |  |  |  |  |
Common stock, issued | 44,514 | 72,777 | 145,461 | 170,930 |  |  |  |  |  |  |  |
Stock repurchased (in US dollars) | $ 0 | $ 26,023,000 | $ 0 | $ 60,508,000 |  |  |  |  |  |  |  |
Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Number of shares repurchased |  |  | 0 |  | 15,900,000 |  |  |  |  |  |  |
Stock repurchased (in US dollars) |  |  |  |  | $ 791,500,000 |  |  |  |  |  |  |
Remaining authorized repurchase amount (in US dollars) | $ 158,500,000 |  | $ 158,500,000 |  | $ 158,500,000 |  |  |  |  |  |  |
Common Stock [Member] | RepurchaseProgramSep262011 |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  |  |  |  |  | $ 100,000,000.0
Common Stock [Member] | RepurchaseProgramDec052013 |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  |  |  |  | $ 100,000,000.0 |
Common Stock [Member] | RepurchaseProgramOct232014 |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  |  |  | $ 250,000,000.0 |  |
Common Stock [Member] | RepurchaseProgramOct282015 |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  |  | $ 200,000,000.0 |  |  |
Common Stock [Member] | RepurchaseProgramOct252017 |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  |  | $ 100,000,000.0 |  |  |  |
Common Stock [Member] | RepurchaseProgramOct232018 |  |  |  |  |  |  |  |  |  |  |  |
Equity, Class of Treasury Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase, authorized amount (in US dollars) |  |  |  |  |  |  | $ 200,000,000.0 |  |  |  |  |

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