Commitments and contingencies - Mortgage Loans (Details)	Jun. 30, 2018 USD ($)
Commitments and Contingencies Disclosure [Abstract]
Within 1 year	$ 3,071,861
1 to 2 years	105,087
2 - 3 years	109,547
3 - 4 years	114,196
5 years and thereafter	3,663,486
Total	$ 7,066,177