Accounting Policies Recently Adopted and Pending Accounting Pronouncements - Additional Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Corporate income tax rate	21.00%		35.00%
Stranded tax effects, reclassify from accumulated other comprehensive loss to retained earnings	$ 8,500,000		$ 8,500,000
Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Percentage of revenues earned	23.00%
Cumulative adjustment	$ 0
Accounting Standards Update 2014-09 [Member] | Before adoption of Topic 606 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Allowance for doubtful accounts related to commission revenue subject to clawback provisions	0
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Total outstanding operating lease obligation			$ 67,900,000
Percentage of total outstanding operating lease obligation of total assets			0.50%
Other Expense [Member] | Accounting Standard Update 2017-07 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Net periodic benefit cost	$ 885,000	$ 1,900,000