Fair value - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Jul. 02, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets and liabilities measured at fair value	$ 2,500,000,000		$ 2,600,000,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes	$ 130,000,000		$ 131,800,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes (percentage)	5.30%		5.00%
Senior notes	$ 172,486,000		$ 172,418,000
Estimated fair value of senior notes	167,200,000
Recurring | Significant Unobservable Inputs (Level 3) | Equities
Purchases and sales of level 3 assets
Purchases	3,300,000	$ 37,800,000
Sales		7,300,000
Recurring | Significant Unobservable Inputs (Level 3) | Fixed maturities | Corporate bonds
Purchases and sales of level 3 assets
Purchases	32,000
Sales		89,900
Recurring | Significant Unobservable Inputs (Level 3) | Term loans
Purchases and sales of level 3 assets
Purchases		237,000
Sales	$ 74,000	$ 74,000
Senior notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Face amount of senior notes				$ 175,000,000
Stated interest rate (percent)	6.50%			6.50%