Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 552.7	$ 524.8	$ 629.4	$ 379.8	$ 373.7	$ 365.5	$ 373.9	$ 361.3	$ 2,086.7	$ 1,474.4	$ 1,405.0
Out-of-pocket reimbursements	41.2	37.8	26.8	25.7	25.1	21.2	16.6	19.4	131.5	82.3	69.0
Total revenues	593.9	562.6	656.2	405.5	398.8	386.7	390.5	380.7	2,218.2	1,556.7	1,474.0
Costs and expenses	481.3	472.1	525.2	326.4	296.5	289.6	320.4	306.9	1,805.0	1,213.4	1,150.2
Depreciation and amortization	35.3	34.7	34.8	23.2	26.9	22.8	24.2	22.1	128.0	96.0	91.1
Operating income	77.3	55.8	96.2	55.9	75.4	74.3	45.9	51.7	285.2	247.3	232.7
Interest expense	(7.1)	(6.9)	(6.9)	(5.9)	(5.5)	(5.4)	(6.5)	(6.1)	(26.8)	(23.5)	(23.8)
Gain on sale of business					5.5	0.0	0.0	0.0	0.0	5.5	0.0
Other income, net	1.8	8.7	15.5	193.0	2.7	6.7	7.0	6.3	219.0	22.7	204.5
Equity in earnings of unconsolidated affiliates	27.2	4.2	3.9	19.2	3.3	7.0	10.2	6.7	54.5	27.2	45.4
Income from continuing operations before income taxes and non-controlling interest	99.2	61.8	108.7	262.2	81.4	82.6	56.6	58.6	531.9	279.2	458.8
Income taxes	18.0	13.0	35.2	18.1	27.3	31.6	22.1	20.1	84.3	101.1	149.2
Income from continuing operations before non-controlling interest	81.2	48.8	73.5	244.1	54.1	51.0	34.5	38.5	447.6	178.1	309.6
Income from discontinued operations, net of tax	0.0	(0.3)	1.9	2.9	(11.7)	222.8	18.7	18.5	4.5	248.3	48.5
Net income	81.2	48.5	75.4	247.0	42.4	273.8	53.2	57.0	452.1	426.4	358.1
Net (income) loss attributable to non-controlling interest	0.0	0.0	0.0	(0.6)	0.5	(0.5)	(0.2)	1.1	(0.6)	0.9	0.1
Net income attributable to DST Systems, Inc.	$ 81.2	$ 48.5	$ 75.4	$ 246.4	$ 42.9	$ 273.3	$ 53.0	$ 58.1	$ 451.5	$ 427.3	$ 358.2
Weighted average common shares outstanding	59.9	60.7	61.9	63.1	64.2	65.4	66.6	67.6	61.4	66.0	72.0
Weighted average diluted shares outstanding	60.6	61.6	62.5	63.9	65.0	66.1	67.2	68.5	62.1	66.6	72.9
Basic earnings per share
Continuing operations attributable to DST Systems, Inc., per basic share (in dollars per share)	$ 1.36	$ 0.81	$ 1.19	$ 3.86	$ 0.85	$ 0.77	$ 0.52	$ 0.59	$ 7.28	$ 2.71	$ 4.30
Discontinued operation, per basic share (in dollars per share)	0.00	(0.01)	0.03	0.05	(0.18)	3.41	0.28	0.27	0.07	3.76	0.67
Basic earnings per share (in dollars per share)	1.36	0.80	1.22	3.91	0.67	4.18	0.80	0.86	7.35	6.47	4.97
Diluted earnings per share
Continuing operations attributable to DST Systems, Inc., per diluted share (in dollars per share)	1.34	0.79	1.18	3.81	0.84	0.76	0.51	0.59	7.20	2.68	4.25
Discontinued operation, per diluted share (in dollars per share)	0.00	0.00	0.03	0.04	(0.18)	3.37	0.28	0.26	0.07	3.73	0.67
Diluted earnings per share (in dollars per share)	1.34	0.79	1.21	3.85	0.66	4.13	0.79	0.85	7.27	6.41	4.92
Cash dividends per share of common stock (in dollars per share)	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.16	$ 0.17	$ 0.17	$ 0.16	$ 0.72	$ 0.66	$ 0.60