Fair value measurements - Significant unobservable inputs (Details)	Sep. 30, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares
Significant Unobservable Inputs
Equity securities	$ 158,700,000	$ 405,200,000
Derivative assets	7,700,000	11,400,000
Derivative liabilities	(3,600,000)	(9,500,000)
Contingent consideration	(300,000)	(28,200,000)
Level 3 Inputs
Significant Unobservable Inputs
Equity securities	0	0
Derivative assets	1,100,000	10,100,000
Derivative liabilities	(3,500,000)	(9,300,000)
Contingent consideration	(300,000)	(28,200,000)
Level 3 Inputs | Private debt instrument
Significant Unobservable Inputs
Debt securities, trading	6,200,000
Level 3 Inputs | Share price of recent transaction | Private equity securities
Significant Unobservable Inputs
Equity securities		32,500,000
Level 3 Inputs | Share price of recent transaction | Preferred stock
Significant Unobservable Inputs
Equity securities		$ 17,500,000
Level 3 Inputs | Share price of recent transaction | Private equity securities
Significant Unobservable Inputs
Equity securities	$ 4,700,000
Level 3 Inputs | Share price of recent transaction | Purchase price | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share) | $ / shares		40.6
Level 3 Inputs | Share price of recent transaction | Purchase price | Preferred stock
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share) | $ / shares		7.74
Level 3 Inputs | Share price of recent transaction | Purchase price | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share) | $ / shares	7.74
Level 3 Inputs | Purchase price of recent transaction | Private equity securities
Significant Unobservable Inputs
Equity securities	$ 32,500,000
Level 3 Inputs | Purchase price of recent transaction | Preferred stock
Significant Unobservable Inputs
Equity securities	27,900,000
Level 3 Inputs | Purchase price of recent transaction | Loan participation
Significant Unobservable Inputs
Equity securities	18,500,000	$ 20,000,000.0
Level 3 Inputs | Purchase price of recent transaction | Loan participation
Significant Unobservable Inputs
Equity securities	10,000,000.0
Level 3 Inputs | Purchase price of recent transaction | Private debt instrument
Significant Unobservable Inputs
Debt securities, trading		7,200,000
Level 3 Inputs | Purchase price of recent transaction | Private equity securities
Significant Unobservable Inputs
Equity securities		1,000,000.0
Level 3 Inputs | Purchase price of recent transaction | private equity securities
Significant Unobservable Inputs
Equity securities		300,000
Level 3 Inputs | Purchase price of recent transaction | Preferred stock
Significant Unobservable Inputs
Equity securities		12,200,000
Derivative assets	1,900,000
Level 3 Inputs | Purchase price of recent transaction | Preferred stock
Significant Unobservable Inputs
Equity securities	700,000	4,800,000
Level 3 Inputs | Purchase price of recent transaction | Private equity funds
Significant Unobservable Inputs
Equity securities	$ 300,000	$ 5,100,000
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share) | $ / shares	40.63
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Preferred stock
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share) | $ / shares	7.74
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Loan participation
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share) | $ / shares		20,000,000.0
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Private debt instrument
Significant Unobservable Inputs
Private debt instrument, measurement input, value		$ 9,000,000.0
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share) | $ / shares		10.0
Level 3 Inputs | Purchase price of recent transaction | Purchase price | private equity securities
Significant Unobservable Inputs
Equity securities, measurement input, value		$ 300,000
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Preferred stock
Significant Unobservable Inputs
Derivative assets, measurement input, value	$ 1,900,000
Equity securities, measurement input, value		12,200,000
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Preferred stock
Significant Unobservable Inputs
Equity securities, measurement input, value	700,000	$ 4,800,000
Level 3 Inputs | Purchase price of recent transaction | Purchase price | Private equity funds
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share) | $ / shares		7.74
Equity securities, measurement input, value	300,000
Level 3 Inputs | Multiple of GAAP book value | Private equity securities
Significant Unobservable Inputs
Equity securities	13,300,000	$ 14,200,000
Level 3 Inputs | Multiple of GAAP book value | Book value multiple | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share) | $ / shares		0.9
Level 3 Inputs | Unit price of recent transaction | Private convertible debt instrument
Significant Unobservable Inputs
Equity securities	2,000,000.0
Level 3 Inputs | Unit price of recent transaction | Purchase price | Private convertible debt instrument
Significant Unobservable Inputs
Equity securities, measurement input, value	7.74
Level 3 Inputs | Option pricing model | Equity warrants
Significant Unobservable Inputs
Equity securities		$ 400,000
Derivative assets	500,000
Level 3 Inputs | Option pricing model | Strike price | Equity warrants
Significant Unobservable Inputs
Derivative assets, measurement input, value	200,000
Equity securities, measurement input, value		200,000
Level 3 Inputs | Third party appraisal | Foreign currency swaps
Significant Unobservable Inputs
Derivative liabilities		(3,600,000)
Level 3 Inputs | Third party appraisal | Foreign currency swaps | Foreign currency swaps
Significant Unobservable Inputs
Equity securities	300,000
Level 3 Inputs | Third party appraisal | Foreign currency forwards
Significant Unobservable Inputs
Derivative assets		2,700,000
Level 3 Inputs | Third party appraisal | Weather derivatives
Significant Unobservable Inputs
Derivative assets		7,000,000.0
Contingent consideration	(3,500,000)
Level 3 Inputs | Third party appraisal | Currency forwards
Significant Unobservable Inputs
Derivative liabilities		(5,700,000)
Level 3 Inputs | Third party appraisal | Currency forwards | Foreign currency forwards
Significant Unobservable Inputs
Equity securities	300,000
Level 3 Inputs | Third party appraisal | Broker quote | Foreign currency swaps
Significant Unobservable Inputs
Derivative liabilities, measurement input, value		(3,600,000)
Level 3 Inputs | Third party appraisal | Broker quote | Foreign currency swaps | Foreign currency swaps
Significant Unobservable Inputs
Equity securities, measurement input, value	300,000
Level 3 Inputs | Third party appraisal | Broker quote | Foreign currency forwards
Significant Unobservable Inputs
Derivative assets, measurement input, value		2,700,000
Level 3 Inputs | Third party appraisal | Broker quote | Weather derivatives
Significant Unobservable Inputs
Derivative assets, measurement input, value		7,000,000.0
Contingent consideration, measurement input, value	(3,500,000)
Level 3 Inputs | Third party appraisal | Broker quote | Currency forwards
Significant Unobservable Inputs
Derivative liabilities, measurement input, value		(5,700,000)
Level 3 Inputs | Third party appraisal | Broker quote | Currency forwards | Foreign currency forwards
Significant Unobservable Inputs
Equity securities, measurement input, value	300,000
Level 3 Inputs | External valuation model
Significant Unobservable Inputs
Derivative liabilities	(300,000)
Level 3 Inputs | External valuation model | Contingent consideration
Significant Unobservable Inputs
Contingent consideration		(28,200,000)
Level 3 Inputs | External valuation model | Discounted future payment
Significant Unobservable Inputs
Derivative liabilities, measurement input, value	$ (300,000)
Level 3 Inputs | External valuation model | Discounted future payment | Contingent consideration
Significant Unobservable Inputs
Contingent consideration, measurement input, value		$ (28,200,000)
Minimum | Level 3 Inputs | Discount yield | Private debt instrument
Significant Unobservable Inputs
Debt securities, trading, measurement input	0.1187
Minimum | Level 3 Inputs | Purchase price of recent transaction | Comparability yields | Loan participation
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share)	0.0446
Minimum | Level 3 Inputs | Purchase price of recent transaction | Purchase price | Loan participation
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share)	0.0446
Minimum | Level 3 Inputs | Multiple of GAAP book value | Book value multiple | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share)	0.73
Maximum | Level 3 Inputs | Discount yield | Private debt instrument
Significant Unobservable Inputs
Debt securities, trading, measurement input	0.1232
Maximum | Level 3 Inputs | Purchase price of recent transaction | Comparability yields | Loan participation
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share)	0.0782
Maximum | Level 3 Inputs | Purchase price of recent transaction | Purchase price | Loan participation
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share)	0.0782
Maximum | Level 3 Inputs | Multiple of GAAP book value | Book value multiple | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share)	0.91
Median | Level 3 Inputs | Discount yield | Private debt instrument
Significant Unobservable Inputs
Debt securities, trading, measurement input	0.1208
Median | Level 3 Inputs | Purchase price of recent transaction | Comparability yields | Loan participation
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share)	0.0574
Median | Level 3 Inputs | Purchase price of recent transaction | Purchase price | Loan participation
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share)	0.0574
Median | Level 3 Inputs | Multiple of GAAP book value | Book value multiple | Private equity securities
Significant Unobservable Inputs
Equity securities, measurement input (in dollars per share)	0.82