Subsequent Events - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended			0 Months Ended	2 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 28, 2015	Mar. 31, 2015	Feb. 09, 2015
Subsequent Event [Line Items]
Dividends declared per common share (usd per share)	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 1.80	$ 1.80	$ 1.80
Subsequent Event
Subsequent Event [Line Items]
Dividends declared per common share (usd per share)												$ 0.45
Dividend payable date												Mar. 18, 2015
Dividend record date												Mar. 04, 2015
Line of Credit
Subsequent Event [Line Items]
Revolving loan facility, borrowing capacity	$ 45,000,000								$ 45,000,000
Line of Credit | Subsequent Event
Subsequent Event [Line Items]
Revolving loan facility, borrowing capacity														45,000,000.0
Amount outstanding on letters of credit														50,000,000.0
Cogent Partners, LP | Scenario, Forecast
Subsequent Event [Line Items]
Payments to Acquire Businesses, Gross													44,000,000
Business Acquisition, Equity Interest Issued or Issuable, Value Assigned													779,460
Payments to Acquire Businesses, Possible Contingent Consideration													$ 18,900,000
Business Acquisition, Contingent Consideration, Possible Number of Shares to be Issued													334,054