Business Segments (Reconciliation of Segment Results to Consolidated Income Before Income Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Segments [Abstract]
Income from construction operations	$ 59,343	$ 49,072	$ 34,045	$ 37,017	$ 52,050	$ 60,919	$ 48,829	$ 40,122	$ 179,477	$ 201,920	$ 105,413
Other income, net									43,882	6,977	13,569
Interest expense									(69,384)	(59,782)	(45,143)
INCOME BEFORE INCOME TAXES	$ 45,193	$ 34,396	$ 52,516	$ 21,870	$ 38,685	$ 47,926	$ 35,780	$ 26,724	$ 153,975	$ 149,115	$ 73,839