Basis of presentation and significant concentrations and risks (Cash Concentration) (Details)	Mar. 31, 2019 USD ($)	Mar. 31, 2019 CNY (¥)	Mar. 31, 2019 HKD ($)	Mar. 31, 2019 MOP (MOP$)	Dec. 31, 2018 USD ($)
Restricted Cash and Cash Equivalents Items [Line Items]
Cash balance insured by the government authority	$ 1,509,352				$ 1,442,481
Restricted cash	427,321,731				325,690,023
RMB denominated bank deposits with Financial Institutions in the PRC [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash, cash equivalents, restricted cash and time deposits maintained at banks	505,806,079				366,773,172
Cash balance insured by the government authority | ¥		¥ 500,000
Renminbi ("RMB") denominated bank deposits with Financial Institutions in Hong Kong Special Administrative Region ("Hong Kong SAR")[Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash, cash equivalents, restricted cash and time deposits maintained at banks	8,134				8,134
Renminbi ("RMB") denominated bank deposits with Financial Institutions in Dubai, UAE [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash, cash equivalents, restricted cash and time deposits maintained at banks	59
U.S. dollar denominated bank deposits with Financial Institution in the U.S. [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash, cash equivalents, restricted cash and time deposits maintained at banks	6,131				40,390
U.S. dollar denominated bank deposits with Financial Institution in the PRC [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash, cash equivalents, restricted cash and time deposits maintained at banks	17,052				17,050
U.S. dollar denominated bank deposits with Financial Institution in Hong Kong SAR [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash, cash equivalents, restricted cash and time deposits maintained at banks	32,299				131,892
Cash balance insured by the government authority			$ 500,000
U.S. dollar denominated bank deposits with Financial Institution in Macau Special Administrative Regio [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash, cash equivalents, restricted cash and time deposits maintained at banks	6,095				6,144
Cash balance insured by the government authority | MOP$				MOP$ 500,000
U.S. dollar denominated bank deposits with Financial Institution in Dubai, UAE [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash, cash equivalents, restricted cash and time deposits maintained at banks	581				14,464
HK dollar denominated bank deposits with Financial institution in Hong Kong SAR [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash, cash equivalents, restricted cash and time deposits maintained at banks	156				156
Dirham denominated bank deposits with Financial Institution in Dubai, UAE [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Cash, cash equivalents, restricted cash and time deposits maintained at banks	329,725				438
Collateral For Bills Payable Related To Purchase Of Raw Materials [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	300,296,989				202,568,664
Syndicated Loans Obtained from Chartered Bank [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	51,789,252				50,766,123
Repayment to settle of Syndicated Loans Obtained from Standard Chartered Bank [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	45,000,000
Short-term bank loans obtained from Postal Savings Bank of China [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	1,485,112
Pledged as collateral to settle short-term bank loans obtained from Postal Savings Bank of China [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	14,900,000
Letter of guarantee [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	72,250,687				70,885,301
Government grant [Member]
Restricted Cash and Cash Equivalents Items [Line Items]
Restricted cash	$ 1,499,691				$ 1,469,935