Revolving Credit Facility (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2012	Jan. 07, 2016
Line of Credit Facility [Line Items]
Line of credit facility	$ 125.0
Maximum borrowing capacity	$ 150.0
Percentage of subsidiary stock securing debt (in hundredths)	65.00%
Unused commitment fee on the average daily unused portion (in hundredths)	0.25%
Letters of Credit Outstanding, Amount	$ 0.6
Special dividend		$ 50.7	$ 25.7
Alternate base rates, applicable margins [Abstract]
Applicable margin, federal funds rate (in hundredths)	0.50%
Applicable margin, 30-day LIBOR (in hundredths)	2.00%
Subsequent Event [Member]
Alternate base rates, applicable margins [Abstract]
Line of Credit Facility, Fair Value of Amount Outstanding				$ 104.4
LIBOR Borrowings [Member]
Line of Credit Facility [Line Items]
Description of basis for variable rate	LIBOR
Applicable margin on variable rate on borrowings, minimum (in hundredths)	2.00%
Applicable margin on variable rate on borrowings, maximum (in hundredths)	2.75%
Alternate Base Rate Borrowings [Member]
Line of Credit Facility [Line Items]
Description of basis for variable rate	alternate base rate
Applicable margin on variable rate on borrowings, minimum (in hundredths)	0.00%
Applicable margin on variable rate on borrowings, maximum (in hundredths)	0.75%