Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Number, before forfeiture, of shares issued under share-based payment arrangement. Excludes employee stock ownership plan (ESOP).

+ References

No definition available.

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- Definition

Amount of stock repurchase plan authorized.

+ References

No definition available.

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- Definition

Amount remaining of a stock repurchase plan authorized.

+ References

No definition available.

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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

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- Definition

Equity impact of the value of stock that has been repurchased during the period and has not been retired and is not held in treasury. Some state laws may mandate 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.

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Common Stock (Details) - USD ($)	3 Months Ended		6 Months Ended		93 Months Ended
Common Stock (Details) - USD ($)	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