Income Tax - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Computed income tax at statutory rates			$ 19,614	$ 19,281
Differences in tax rates due to multiple jurisdictions			(201)	(466)
Tax benefit due to a change in estimate			0	(334)
Effect of income subject to tax-exemption grant			(12,902)	(12,580)
Unrecognized tax benefit			556	187
Other benefit			(20)	0
Income tax expense	$ 3,112	$ 4,068	$ 7,047	$ 6,088