Investments in debt and equity securities - Amortized Costs and Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Amortized costs	$ 545,734	$ 608,686
Fair values	561,986	602,020
Municipal
Debt Securities, Available-for-sale [Line Items]
Amortized costs	53,315	61,779
Fair values	54,964	61,934
Corporate
Debt Securities, Available-for-sale [Line Items]
Amortized costs	299,355	333,289
Fair values	311,148	328,495
Foreign
Debt Securities, Available-for-sale [Line Items]
Amortized costs	186,523	200,667
Fair values	189,308	198,938
U.S. Treasury Bonds
Debt Securities, Available-for-sale [Line Items]
Amortized costs	6,541	12,951
Fair values	$ 6,566	$ 12,653