Equity - Other Comprehensive Income (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) affiliate	Dec. 31, 2016 USD ($) affiliate	Dec. 31, 2015 USD ($) affiliate
Components of accumulated other comprehensive income (loss)
Balance at beginning of the period	$ 16.6	$ 41.9
Other comprehensive loss, net of tax	(15.2)	(25.3)	$ (145.0)
Balance at end of the period	1.4	16.6	41.9
Available-for-sale securities, Pretax
Unrealized gains (losses) on available-for-sale securities	7.0	18.1	(35.6)
Reclassification of (gains) losses into net earnings on available-for-sale securities	(155.9)	(2.1)	(163.5)
Net change in available-for-sale securities	(148.9)	16.0	(199.1)
Cash flow hedges, Pretax
Unrealized gains (losses) on cash flow hedges	(0.2)	0.1	0.4
Net change in cash flow hedges	0.4	0.1	0.1
Unrealized net gains on defined benefit pension plan	5.2	0.0	0.0
Current Period translation adjustments, pretax	36.0	(34.9)	(22.0)
Reclassification into net earnings upon liquidation of foreign entities, pretax	41.0	0.0	0.0
Cumulative translation adjustments, Pretax	73.7	(35.4)	(22.0)
Other comprehensive loss	(69.6)	(19.3)	(221.0)
Available-for-sale securities, Net of Tax
Unrealized gains (losses) on available-for-sale securities	4.5	11.4	(21.8)
Reclassification of (gains) losses into net earnings on available-for-sale securities	(97.7)	(1.3)	(101.4)
Net change in available-for-sale securities	(93.2)	10.1	(123.2)
Cash flow hedges, Net of Tax
Unrealized gains (losses) on cash flow hedges	(0.1)	0.0	0.4
Net change in cash flow hedges	0.2	0.0	0.2
Unrealized net gains on defined benefit pension plan	4.1	0.0	0.0
Current period translation adjustment, net of tax	36.0	(34.9)	(22.0)
Reclassification into net earnings upon liquidation of foreign entities, net of tax	41.0	0.0	0.0
Cumulative translation adjustments, Net of Tax	73.7	(35.4)	(22.0)
Other comprehensive loss, net of tax	$ (15.2)	$ (25.3)	$ (145.0)
One of DST's unconsolidated affiliates
Cash flow hedges, Net of Tax
Number of unconsolidated affiliates with an interest rate swap | affiliate	1	1	1
Unconsolidated affiliate's interest rate swap liability, fair value	$ 25.5	$ 33.1	$ 44.7
Equity Method Investment, Ownership Percentage	50.00%	50.00%	50.00%
Proportionate share in unconsolidated affiliate's interest rate swap liability	$ 12.8	$ 16.5	$ 22.4
Foreign currency
Cash flow hedges, Pretax
Reclassification of losses (gains) into net earnings on cash flow hedges	0.6	0.0	(0.5)
Cash flow hedges, Net of Tax
Reclassification of losses (gains) into net earnings on cash flow hedges	0.3	0.0	(0.3)
Interest rate
Cash flow hedges, Pretax
Reclassification of losses (gains) into net earnings on cash flow hedges	0.0	0.0	0.2
Cash flow hedges, Net of Tax
Reclassification of losses (gains) into net earnings on cash flow hedges	0.0	0.0	0.1
Available-for-Sale Securities
Components of accumulated other comprehensive income (loss)
Balance at beginning of the period	94.1	84.0
Other comprehensive loss, net of tax	(93.2)	10.1
Balance at end of the period	0.9	94.1	84.0
Cash flow hedges, Net of Tax
Other comprehensive loss, net of tax	(93.2)	10.1
Cash Flow Hedges
Components of accumulated other comprehensive income (loss)
Balance at beginning of the period	(0.1)	(0.1)
Other comprehensive loss, net of tax	0.2	0.0
Balance at end of the period	0.1	(0.1)	(0.1)
Cash flow hedges, Net of Tax
Other comprehensive loss, net of tax	0.2	0.0
Defined Benefit Pension
Components of accumulated other comprehensive income (loss)
Balance at beginning of the period	0.0	0.0
Other comprehensive loss, net of tax	4.1	0.0
Balance at end of the period	4.1	0.0	0.0
Cash flow hedges, Net of Tax
Other comprehensive loss, net of tax	4.1	0.0
Foreign Currency Translation
Components of accumulated other comprehensive income (loss)
Balance at beginning of the period	(77.4)	(42.0)
Other comprehensive loss, net of tax	73.7	(35.4)
Balance at end of the period	(3.7)	(77.4)	(42.0)
Cash flow hedges, Net of Tax
Other comprehensive loss, net of tax	73.7	(35.4)
Discontinued Operations, Held-for-sale
Cash flow hedges, Pretax
Reclassification into net earnings upon liquidation of foreign entities, pretax	3.3	0.5	0.0
Cash flow hedges, Net of Tax
Reclassification into net earnings upon liquidation of foreign entities, net of tax	$ 3.3	$ 0.5	$ 0.0