Segment Information - Net assets and liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
ASSETS
Current assets	$ 688,855	$ 621,543
Total assets	1,312,975	1,336,285
LIABILITIES AND SHAREHOLDERS' EQUITY
Current liabilities	372,070	376,479
Total liabilities and shareholders' equity	1,312,975	$ 1,336,285
P3 Joint Venture
ASSETS
Current assets	11,137
Noncurrent assets	8,407
Total assets	19,544
LIABILITIES AND SHAREHOLDERS' EQUITY
Current liabilities	957
Noncurrent liabilities	18,587
Total liabilities	19,544
Total liabilities and shareholders' equity	$ 19,544