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Equity (Stock Repurchase Program) (Details) - USD ($) $ / shares in Units, shares in Millions | 1 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Equity [Abstract] |  |  |  |
Repurchase of common stock, authorized amount |  | $ 50,000,000.0 |  |
Stock repurchased and retired during period, shares |  |  | 10.2 | 5.7
Common stock value acquired including acquisition costs |  |  | $ 98,400,000 | $ 70,400,000
Average cost per share | $ 11.51 |  | $ 9.67 | $ 12.25
Class of Stock [Line Items] |  |  |  |
Shares repurchased during period | 4.0 |  |  |
Repurchase of stock | $ 45,900,000 | $ 45,928,000 | $ 98,429,000 | $ 70,416,000
3.125% senior convertible notes |  |  |  |
Class of Stock [Line Items] |  |  |  |
Stated interest rate (as a percent) | 3.125% | 3.125% |  |

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- Definition Common Stock Value Acquired Including Acquisition Costs + References No definition available. + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of stock repurchase plan authorized. + References No definition available. + Details
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- Definition Number of shares that have been repurchased and retired during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21463-112644 Reference 4: http://www.xbrl.org/2003/role/presentationRef -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 + Details
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