Fair value measurements - Fair value of other financial instruments not recorded at fair value (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Estimated fair values of financial instruments (other than derivative financial instruments)
Carrying amount - Loans from banks and other parties	$ 657,549	$ 602,083
Level 2
Estimated fair values of financial instruments (other than derivative financial instruments)
Carrying amount - Loans from banks and other parties	657,549	602,083
Fair value - Loans from banks and other parties	$ 658,666	$ 626,613