Fair value - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2020	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	$ 2,600,000,000		$ 2,600,000,000			$ 2,600,000,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes	$ 121,100,000		$ 121,100,000			$ 131,800,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes (percentage)	4.70%		4.70%			5.00%
Senior notes	$ 172,554,000		$ 172,554,000		$ 172,554,000	$ 172,418,000
Estimated fair value of senior notes	176,400,000		176,400,000
Recurring | Significant Unobservable Inputs (Level 3) | Equities
Purchases and sales of level 3 assets
Purchases	9,600,000		12,900,000	$ 37,800,000
Sales	25,100,000		25,100,000	7,300,000
Recurring | Significant Unobservable Inputs (Level 3) | Fixed maturities | Corporate bonds
Purchases and sales of level 3 assets
Purchases	33,000		65,000
Sales				100,000
Recurring | Significant Unobservable Inputs (Level 3) | Term loans
Purchases and sales of level 3 assets
Purchases		$ 200,000		500,000
Sales	$ 7,200,000	$ 100,000	$ 7,300,000	$ 100,000
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%				6.50%