Income Taxes - Reconciliation of the Statutory Federal Income Tax Rate to the Effective Tax Rate (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of the statutory federal income tax rate to the effective tax rate
Statutory federal income tax rate	35.00%	35.00%	35.00%
Effect of:
State and local income taxes	2.20%	2.30%	2.60%
Investment vehicles	(1.40%)	(1.50%)	(1.60%)
Domestic production activities	(3.00%)	(2.90%)	(2.20%)
Employee share-based payments	(5.60%)	(2.80%)
Other - net	(2.10%)	(1.10%)	(1.80%)
Pre-adjusted effective tax rate	25.10%	29.00%	32.00%
Effect of:
Tax Act	(39.80%)
Subsidiary mergers	(4.00%)
Post-adjusted effective tax rate	(18.70%)	29.00%	32.00%

Effective Income Tax Rate Reconciliation, Business Combination, Percent

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Effective Income Tax Rate Reconciliation, Effect Of Significant Events, Percent [Abstract]

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Effective Income Tax Rate Reconciliation, Pre-Adjusted Effective Tax Rate, Percent

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Effective Income Tax Rate Reconciliation, Pre-Adjusted Rate, Percent [Abstract]

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Effective Income Tax Rate Reconciliation, Tax Cuts And Jobs Act Of 2017, Percent

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
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-Section 50
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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deduction for qualified production activity.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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