Variable interest entities - Additional Information (Detail) $ in Thousands	Mar. 31, 2019 USD ($)
Variable Interest Entity [Line Items]
Total assets of variable interest entities	$ 1,118,672
Total liabilities of variable interest entities	673,499
DMG held for sale
Variable Interest Entity [Line Items]
Total assets of variable interest entities	737,309
Total liabilities of variable interest entities	$ 401,308