Tax Loss and Credit Carryforwards (Detail) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
U.S. federal
Tax Credit Carryforward [Line Items]
Tax loss carryforwards		$ 595	$ 483
Tax loss carryforwards, Expiration year		Dec. 31, 2027
State and Local
Tax Credit Carryforward [Line Items]
Tax loss carryforwards		$ 4,362	5,265
Tax loss carryforwards, Expiration year		Dec. 31, 2027
Illinois Edge
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	[1]	$ 8,422	$ 5,045
Tax credit carryforwards, Expiration year	[1]	Dec. 31, 2018

[1] Amounts are before the federal benefit of state tax