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Common Stock - Summary of Information Regarding Outstanding SARs (Detail) - $ / shares | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Weighted-Average Grant Date Fair Value of Restricted Stock, Beginning Balance | $ 41.10 | $ 61.53 | $ 73.73
Granted | 38.74 | 31.59 | 53.27
Forfeited | 55.22 | 49.95 | 62.73
Weighted-Average Grant Date Fair Value of Restricted Stock, Ending Balance | $ 37.04 | $ 41.10 | $ 61.53
Restricted Shares Outstanding, Beginning balance | 4,563,983 | 1,969,250 | 1,569,141
Granted | 1,738,589 | 3,384,325 | 954,075
Forfeited | (394,688) | (224,390) | (148,639)
Restricted Shares Outstanding, Ending balance | 4,889,678 | 4,563,983 | 1,969,250
Stock Appreciation Rights (SARs) [Member] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |
Weighted-Average Grant Date Fair Value of Restricted Stock, Beginning Balance | $ 28.32 |  |
Granted | 38.86 | $ 28.32 |
Forfeited | 28.35 | 28.24 |
Exercised | 34.72 |  |
Weighted-Average Grant Date Fair Value of Restricted Stock, Ending Balance | 28.41 | $ 28.32 |
Average Exercise Price, Exercisable at end of year | $ 28.33 |  |
Restricted Shares Outstanding, Beginning balance | 4,341,740 |  |
Granted | 14,400 | 4,618,400 |
Forfeited | (283,822) | (276,660) |
Exercised | (2,578,629) |  |
Restricted Shares Outstanding, Ending balance | 1,493,689 | 4,341,740 |
Exercisable at end of year | 75,102 |  |

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- Definition Share based compensation arrangement by share based payment award equity instruments other than options exercisable weighted average exercise price. + References No definition available. + Details
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- Definition Share based compensation arrangement by share based payment award equity instruments other than options exercised in period. + References No definition available. + Details
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- Definition Share based compensation arrangement by share based payment award equity instruments other than options exercised in period weighted average grant date fair value. + References No definition available. + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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