Debt - Schedule of Fixed Rate Debt (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Senior notes | Carrying Value
Notes and other indebtedness [Line Items]
Amount outstanding	$ 607.8	$ 330.0
Senior notes | Estimated Fair Value
Notes and other indebtedness [Line Items]
Amount outstanding	613.1	346.3
2010 Senior notes—Series B | Carrying Value
Notes and other indebtedness [Line Items]
Amount outstanding	0.0	105.0
2010 Senior notes—Series B | Estimated Fair Value
Notes and other indebtedness [Line Items]
Amount outstanding	0.0	106.7
2010 Senior notes—Series C | Carrying Value
Notes and other indebtedness [Line Items]
Amount outstanding	65.0	65.0
2010 Senior notes—Series C | Estimated Fair Value
Notes and other indebtedness [Line Items]
Amount outstanding	65.7	67.5
2010 Senior notes—Series D | Carrying Value
Notes and other indebtedness [Line Items]
Amount outstanding	160.0	160.0
2010 Senior notes—Series D | Estimated Fair Value
Notes and other indebtedness [Line Items]
Amount outstanding	167.4	172.1
2017 Senior notes—Series A | Carrying Value
Notes and other indebtedness [Line Items]
Amount outstanding	35.0	0.0
2017 Senior notes—Series A | Estimated Fair Value
Notes and other indebtedness [Line Items]
Amount outstanding	34.6	0.0
2017 Senior notes—Series B | Carrying Value
Notes and other indebtedness [Line Items]
Amount outstanding	105.0	0.0
2017 Senior notes—Series B | Estimated Fair Value
Notes and other indebtedness [Line Items]
Amount outstanding	104.0	0.0
2017 Senior notes—Series D | Carrying Value
Notes and other indebtedness [Line Items]
Amount outstanding	105.0	0.0
2017 Senior notes—Series D | Estimated Fair Value
Notes and other indebtedness [Line Items]
Amount outstanding	104.5	0.0
2017 Senior notes—Series E | Carrying Value
Notes and other indebtedness [Line Items]
Amount outstanding	50.0	0.0
2017 Senior notes—Series E | Estimated Fair Value
Notes and other indebtedness [Line Items]
Amount outstanding	49.6	0.0
2017 Senior notes—Series F | Carrying Value
Notes and other indebtedness [Line Items]
Amount outstanding	55.0	0.0
2017 Senior notes—Series F | Estimated Fair Value
Notes and other indebtedness [Line Items]
Amount outstanding	54.1	0.0
U.K. Mortgage | Carrying Value
Notes and other indebtedness [Line Items]
Amount outstanding	28.4	0.0
U.K. Mortgage | Estimated Fair Value
Notes and other indebtedness [Line Items]
Amount outstanding	28.8	0.0
U.S. Mortgage | Carrying Value
Notes and other indebtedness [Line Items]
Amount outstanding	4.4	0.0
U.S. Mortgage | Estimated Fair Value
Notes and other indebtedness [Line Items]
Amount outstanding	$ 4.4	$ 0.0