Accounting Policies Recently Adopted and Pending Accounting Pronouncements - Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 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
Topic 606 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Percentage of revenues earned			23.00%
Cumulative adjustment	$ 0		$ 0
Topic 606 [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		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	$ 966,000	$ 2,700,000	$ 4,800,000