Investments - Mortgage Loans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 16,164.9	$ 14,398.0	$ 16,164.9	$ 14,398.0		$ 15,364.3
Mortgage loan valuation allowance	(28.1)	(30.1)	(28.1)	(30.1)	$ (27.6)	(27.4)	$ (33.0)	$ (32.7)
Mortgage loans, Total carrying value	16,136.8		16,136.8			15,336.9
Commercial mortgage loans
Mortgage loan disclosures
Mortgage loans, Total amortized cost	14,830.6	13,071.9	14,830.6	13,071.9		13,996.3
Mortgage loan valuation allowance	(25.4)	(25.8)	(25.4)	(25.8)	(24.7)	$ (24.3)	(26.6)	(25.8)
Mortgage loans, purchased	$ 103.0		115.9
Mortgage loans, sold		1.6	$ 0.5	1.6
Percent of mortgage loans (as a percent)	100.00%		100.00%			100.00%
Commercial mortgage loans | Office
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 4,960.6		$ 4,960.6			$ 4,625.8
Percent of mortgage loans (as a percent)	33.40%		33.40%			33.00%
Commercial mortgage loans | Retail
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 2,118.9		$ 2,118.9			$ 2,305.6
Percent of mortgage loans (as a percent)	14.30%		14.30%			16.50%
Commercial mortgage loans | Industrial
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 2,350.5		$ 2,350.5			$ 2,312.9
Percent of mortgage loans (as a percent)	15.80%		15.80%			16.50%
Commercial mortgage loans | Apartments
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 4,850.1		$ 4,850.1			$ 4,250.5
Percent of mortgage loans (as a percent)	32.70%		32.70%			30.40%
Commercial mortgage loans | Hotel
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 97.9		$ 97.9			$ 99.8
Percent of mortgage loans (as a percent)	0.70%		0.70%			0.70%
Commercial mortgage loans | Mixed use/other
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 452.6		$ 452.6			$ 401.7
Percent of mortgage loans (as a percent)	3.10%		3.10%			2.90%
Commercial mortgage loans | New England
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 652.6		$ 652.6			$ 640.6
Percent of mortgage loans (as a percent)	4.40%		4.40%			4.60%
Commercial mortgage loans | Middle Atlantic
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 4,263.9		$ 4,263.9			$ 3,927.3
Percent of mortgage loans (as a percent)	28.70%		28.70%			28.00%
Commercial mortgage loans | East North Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 604.9		$ 604.9			$ 592.8
Percent of mortgage loans (as a percent)	4.10%		4.10%			4.20%
Commercial mortgage loans | West North Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 200.5		$ 200.5			$ 205.8
Percent of mortgage loans (as a percent)	1.40%		1.40%			1.50%
Commercial mortgage loans | South Atlantic
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 2,338.0		$ 2,338.0			$ 2,206.5
Percent of mortgage loans (as a percent)	15.80%		15.80%			15.80%
Commercial mortgage loans | East South Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 444.8		$ 444.8			$ 422.5
Percent of mortgage loans (as a percent)	3.00%		3.00%			3.00%
Commercial mortgage loans | West South Central
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,463.1		$ 1,463.1			$ 1,213.8
Percent of mortgage loans (as a percent)	9.90%		9.90%			8.70%
Commercial mortgage loans | Mountain
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 897.2		$ 897.2			$ 968.6
Percent of mortgage loans (as a percent)	6.00%		6.00%			6.90%
Commercial mortgage loans | Pacific
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 3,686.2		$ 3,686.2			$ 3,567.6
Percent of mortgage loans (as a percent)	24.90%		24.90%			25.50%
Commercial mortgage loans | International
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 279.4		$ 279.4			$ 250.8
Percent of mortgage loans (as a percent)	1.80%		1.80%			1.80%
Residential mortgage loans
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 1,334.3	1,326.1	$ 1,334.3	1,326.1		$ 1,368.0
Mortgage loan valuation allowance	(2.7)	(4.3)	(2.7)	(4.3)	$ (2.9)	(3.1)	$ (6.4)	$ (6.9)
Mortgage loans, purchased	66.2	92.6	99.4	204.8
Mortgage loans, sold	21.2	$ 27.7	32.1	$ 51.3
First liens
Mortgage loan disclosures
Mortgage loans, Total amortized cost	1,321.6		1,321.6			1,352.9
Home equity
Mortgage loan disclosures
Mortgage loans, Total amortized cost	$ 12.7		$ 12.7			$ 15.1