Selected Quarterly Financial Data - Schedule of Quarterly Financial Data (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended								12 Months Ended
	Jan. 01, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Revenue		$ 332.4	$ 313.8	$ 273.9	$ 258.9	$ 262.7	$ 261.3	$ 252.4	$ 243.5	$ 1,179.0	$ 1,019.9	$ 911.7
Total operating expense		292.7	264.2	223.1	209.4	213.4	195.9	198.8	196.0	989.4	804.1	741.9
Operating income		39.7	49.6	50.8	49.5	49.3	65.4	53.6	47.5	189.6	215.8	169.8
Non-operating income (expense), net		(4.0)	13.9	2.3	(3.3)	(0.9)	7.3	1.4	9.3	8.9	17.1	11.3
Income before income taxes and equity in net income (loss) of unconsolidated entities		35.7	63.5	53.1	46.2	48.4	72.7	55.0	56.8	198.5	232.9	181.1
Equity in net income (loss) of unconsolidated entities		1.0	(1.1)	0.7	(1.5)	(0.5)	0.3	(0.4)	(1.5)	(0.9)	(2.1)	(1.3)
Income tax expense		$ 9.1	$ 13.3	$ 11.7	$ 11.5	$ 5.5	$ 16.1	$ 12.8	$ 13.4	45.6	47.8	42.9
Consolidated net income										$ 152.0	$ 183.0	$ 136.9
Earnings Per Share, Basic and Diluted [Abstract]
Basic (in dollars per share)		$ 0.64	$ 1.15	$ 0.99	$ 0.78	$ 0.99	$ 1.33	$ 0.98	$ 0.99
Diluted (in dollars per share)		0.64	1.14	0.98	0.77	0.99	1.32	0.97	0.98
Dividends Per Common Share: [Abstract]
Common stock, dividends, per share, declared (in dollars per share)		0.58	0	0.28	0.28	0.53	0	0.25	0.25	$ 1.14	$ 1.03	$ 0.94
Dividends paid per common share (in dollars per share)		$ 0.28	$ 0.28	$ 0.28	$ 0.28	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 1.12	$ 1.00	$ 0.92
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Weighted average common shares outstanding - basic (in shares)		42.8	42.8	42.7	42.6	42.7	42.6	42.6	42.5	42.7	42.6	42.7
Weighted average common shares outstanding - diluted (in shares)		43.3	43.2	43.1	43.0	43.1	43.1	43.0	42.9	43.2	43.0	43.0
Revenue benefit from amended license agreement							$ 10.5
Gain on sale of a product line										$ 0.0	$ 10.5	$ 0.0
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Board Consulting Services Product Line
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]
Gain on sale of a product line	$ 10.5								$ 10.5