Equity method investment - Financial Position and Results of Operations (Details) - Bankers Healthcare Group, LLC - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Assets	$ 353,362		$ 353,362		$ 330,030
Liabilities	248,881		248,881		224,837
Membership interests	104,481		104,481		105,193
Total liabilities and membership interests	353,362		353,362		$ 330,030
Equity Method Investment, Summarized Financial Information, Income Statement [Abstract]
Revenues	49,053	$ 37,012	92,804	$ 71,247
Net income	$ 19,102	$ 18,013	$ 38,106	$ 34,024

The amount of assets reported by an equity method investment of the entity.

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Amount of equity, including noncontrolling interest, reported by an equity method investment of the entity.

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Amount of income (loss) from continuing operations reported by an equity method investee.

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The amount of liabilities reported by an equity method investment of the entity.

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The amount of liabilities and equity reported by an equity method investment of the entity.

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The amount of revenue from sale of goods and services reduced by sales returns, allowances, and discounts reported by an equity method investment of the entity.

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