Dividend restrictions - Additional Information (Details) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Disclosure of Restrictions on Dividends, Loans and Advances Disclosure [Abstract]
Amount available for payment of dividend without prior approval	$ 164,859,000	$ 105,453,000
Dividends declared	$ 0	$ 0

Amount of paid and unpaid cash dividends declared for classes of stock, for example, but not limited to, common and preferred.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6957935&loc=d3e64057-112817

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration