Fair value measurements - Significant Unobservable Inputs (Details) $ in Millions	Dec. 31, 2018 USD ($) $ / shares	Dec. 31, 2017 USD ($) $ / shares
Significant Unobservable Inputs
Equity securities	$ 380.0	$ 299.2
Derivative assets	4.1	4.5
Derivative liabilities	(5.1)	(10.6)
Contingent consideration	(28.8)	(42.8)
Level 3
Significant Unobservable Inputs
Derivative assets	4.1	4.5
Derivative liabilities	(4.6)	(10.6)
Contingent consideration	(28.8)	(42.8)
Level 3 | Share price of recent transaction | Private equity securities
Significant Unobservable Inputs
Equity securities	32.5	25.0
Level 3 | Share price of recent transaction | Preferred stocks
Significant Unobservable Inputs
Equity securities	4.6	$ 2.0
Level 3 | Share price of recent transaction | Preferred stock 1
Significant Unobservable Inputs
Equity securities	0.8
Level 3 | Share price of recent transaction | Common shares
Significant Unobservable Inputs
Equity securities	$ 0.3
Level 3 | Share price of recent transaction | Purchase share price | Private equity securities
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	40.63	31.25
Level 3 | Share price of recent transaction | Purchase price | Preferred stocks
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	1.88
Level 3 | Share price of recent transaction | Purchase price | Preferred stock 1
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	0.80
Level 3 | Share price of recent transaction | Purchase price | Common shares
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	10.0
Level 3 | Share price of recent transaction | Average share price | Preferred stocks
Significant Unobservable Inputs
Equity Securities, Input | $ / shares		11.79
Level 3 | Multiple of GAAP book value | Private equity securities
Significant Unobservable Inputs
Equity securities	$ 14.7	$ 17.2
Level 3 | Multiple of GAAP book value | Book value multiple | Private equity securities
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	0.9	1.0
Level 3 | Purchase price of recent transaction | Private debt instrument
Significant Unobservable Inputs
Private debt instrument	$ 9.0
Level 3 | Purchase price of recent transaction | Private debt instrument 1
Significant Unobservable Inputs
Private debt instrument	6.0	$ 9.0
Level 3 | Purchase price of recent transaction | Private debt instrument 2
Significant Unobservable Inputs
Private debt instrument		9.0
Level 3 | Purchase price of recent transaction | Private debt instrument 3
Significant Unobservable Inputs
Private debt instrument		1.5
Level 3 | Purchase price of recent transaction | Preferred stocks
Significant Unobservable Inputs
Equity securities		$ 6.0
Level 3 | Purchase price of recent transaction | Common shares
Significant Unobservable Inputs
Equity securities	0.9
Level 3 | Purchase price of recent transaction | Interest rate cap | Private debt instrument 2
Significant Unobservable Inputs
Private debt instrument	$ 0.2
Level 3 | Purchase price of recent transaction | Purchase price | Private debt instrument
Significant Unobservable Inputs
Debt instrument, Input | $ / shares	9.0
Level 3 | Purchase price of recent transaction | Purchase price | Private debt instrument 1
Significant Unobservable Inputs
Debt instrument, Input | $ / shares	6.0	9.0
Level 3 | Purchase price of recent transaction | Purchase price | Private debt instrument 2
Significant Unobservable Inputs
Debt instrument, Input | $ / shares		9.0
Level 3 | Purchase price of recent transaction | Purchase price | Private debt instrument 3
Significant Unobservable Inputs
Debt instrument, Input | $ / shares		1.50
Level 3 | Purchase price of recent transaction | Purchase price | Common shares
Significant Unobservable Inputs
Equity Securities, Input | $ / shares	1.88
Level 3 | Purchase price of recent transaction | Purchase price | Interest rate cap | Private debt instrument 2
Significant Unobservable Inputs
Debt instrument, Input | $ / shares	0.2
Level 3 | Purchase price of recent transaction | Average share price | Preferred stocks
Significant Unobservable Inputs
Equity Securities, Input | $ / shares		0.6
Level 3 | Internal valuation model | Private debt instrument
Significant Unobservable Inputs
Private debt instrument		$ 2.5
Level 3 | Internal valuation model | Purchase price less pay down | Private debt instrument
Significant Unobservable Inputs
Debt instrument, Input | $ / shares		2.5
Level 3 | Third party appraisal | Weather derivatives
Significant Unobservable Inputs
Derivative assets	$ 3.9	$ 4.2
Level 3 | Third party appraisal | Interest rate cap
Significant Unobservable Inputs
Derivative assets	0.2	0.3
Level 3 | Third party appraisal | Foreign currency swaps
Significant Unobservable Inputs
Derivative liabilities	$ (4.6)	$ (10.6)
Level 3 | Third party appraisal | Broker quote | Weather derivatives
Significant Unobservable Inputs
Derivative assets, Input | $ / shares	3.9	4.2
Level 3 | Third party appraisal | Broker quote | Interest rate cap
Significant Unobservable Inputs
Derivative assets, Input | $ / shares	0.2	0.3
Level 3 | Third party appraisal | Broker quote | Foreign currency swaps
Significant Unobservable Inputs
Derivative liabilities, Input | $ / shares	(4.6)	(10.6)
Level 3 | External valuation model
Significant Unobservable Inputs
Contingent consideration	$ (28.8)	$ (42.8)
Level 3 | External valuation model | Discounted future payments
Significant Unobservable Inputs
Contingent consideration, Input | $ / shares	(28.8)	(42.80)