Fair value measurements - Significant Unobservable Inputs (Details) $ in Millions	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares
Significant Unobservable Inputs
Equity securities	$ 394.4	$ 380.0
Derivative assets	0.8	4.1
Derivative liabilities	(5.3)	(5.1)
Contingent consideration	(28.8)	(28.8)
Level 3
Significant Unobservable Inputs
Derivative assets	0.3	4.1
Derivative liabilities	(5.1)	(4.6)
Contingent consideration	(28.8)	(28.8)
Level 3 | Share price of recent transaction | Private equity securities
Significant Unobservable Inputs
Equity securities	32.5	32.5
Level 3 | Share price of recent transaction | Private equity securities 1
Significant Unobservable Inputs
Equity securities	5.1	0.3
Level 3 | Share price of recent transaction | Preferred stocks
Significant Unobservable Inputs
Equity securities	$ 17.5	4.6
Level 3 | Share price of recent transaction | Preferred stock 1
Significant Unobservable Inputs
Equity securities		$ 0.8
Level 3 | Share price of recent transaction | Purchase share price | Private equity securities
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	40.63	40.63
Level 3 | Share price of recent transaction | Purchase price | Private equity securities 1
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	7.74	10.00
Level 3 | Share price of recent transaction | Purchase price | Preferred stocks
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	7.74	1.88
Level 3 | Share price of recent transaction | Purchase price | Preferred stock 1
Significant Unobservable Inputs
Equity Securities, Input | $ / shares		0.8
Level 3 | Multiple of GAAP book value | Private equity securities
Significant Unobservable Inputs
Equity securities	$ 16.8	$ 14.7
Level 3 | Multiple of GAAP book value | Book value multiple | Private equity securities
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	1.0	0.9
Level 3 | Purchase price of recent transaction | Private equity securities
Significant Unobservable Inputs
Equity securities	$ 1.0	$ 0.9
Level 3 | Purchase price of recent transaction | Private equity securities 1
Significant Unobservable Inputs
Equity securities	0.8
Level 3 | Purchase price of recent transaction | Private equity securities 2
Significant Unobservable Inputs
Equity securities	0.3
Level 3 | Purchase price of recent transaction | Private debt instrument
Significant Unobservable Inputs
Private debt instrument	7.8	9.0
Level 3 | Purchase price of recent transaction | Private debt instrument 1
Significant Unobservable Inputs
Private debt instrument	6.0	6.0
Level 3 | Purchase price of recent transaction | Private debt instrument 2
Significant Unobservable Inputs
Private debt instrument	$ 0.2
Level 3 | Purchase price of recent transaction | Private debt instrument 3
Significant Unobservable Inputs
Private debt instrument		$ 0.2
Level 3 | Purchase price of recent transaction | Purchase price | Private equity securities
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	10.0	1.88
Level 3 | Purchase price of recent transaction | Purchase price | Private equity securities 1
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	0.8
Level 3 | Purchase price of recent transaction | Purchase price | Private equity securities 2
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	0.30
Level 3 | Purchase price of recent transaction | Purchase price | Private debt instrument
Significant Unobservable Inputs
Debt instrument, Input | $ / shares	9.0	9.0
Level 3 | Purchase price of recent transaction | Purchase price | Private debt instrument 1
Significant Unobservable Inputs
Debt instrument, Input | $ / shares	6.00	6.0
Level 3 | Purchase price of recent transaction | Purchase price | Private debt instrument 2
Significant Unobservable Inputs
Debt instrument, Input | $ / shares	0.2
Level 3 | Purchase price of recent transaction | Purchase price | Private debt instrument 3
Significant Unobservable Inputs
Debt instrument, Input | $ / shares		0.2
Level 3 | Third party appraisal | Weather derivatives
Significant Unobservable Inputs
Derivative assets		$ 3.9
Derivative liabilities	$ (1.3)
Level 3 | Third party appraisal | Interest rate cap
Significant Unobservable Inputs
Derivative assets	0.1	0.2
Level 3 | Third party appraisal | Foreign currency swaps
Significant Unobservable Inputs
Derivative liabilities	(3.8)	$ (4.6)
Level 3 | Third party appraisal | Effective yield | Foreign currency forwards
Significant Unobservable Inputs
Derivative assets	$ 0.2
Level 3 | Third party appraisal | Broker quote | Foreign currency forwards
Significant Unobservable Inputs
Derivative assets, Input | $ / shares	0.2
Level 3 | Third party appraisal | Broker quote | Weather derivatives
Significant Unobservable Inputs
Derivative assets, Input | $ / shares		3.9
Derivative liabilities, Input | $ / shares	(1.3)
Level 3 | Third party appraisal | Broker quote | Interest rate cap
Significant Unobservable Inputs
Derivative assets, Input | $ / shares	0.1	0.2
Level 3 | Third party appraisal | Broker quote | Foreign currency swaps
Significant Unobservable Inputs
Derivative liabilities, Input | $ / shares	(3.8)	(4.60)
Level 3 | External valuation model
Significant Unobservable Inputs
Contingent consideration	$ (28.8)	$ (28.8)
Level 3 | External valuation model | Discounted future payments
Significant Unobservable Inputs
Contingent consideration, Input | $ / shares	(28.80)	(28.80)