Condensed Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($)	Total	Common Stock [Member]	Capital In Excess of Par Value [Member]	Accumulated other comprehensive income [Member]	Retained earnings [Member]	Non-controlling interests in consolidated subsidiaries [Member]
Stockholders' equity, beginning balance at Dec. 31, 2016	$ 1,194,070,000	$ 10,016,000	$ 502,500,000	$ 0	$ 681,554,000	$ 0
Net income	24,988,000	0	0	0	24,988,000	0
Other comprehensive income (loss)	20,000	0	0	20,000	0	0
Total comprehensive income	25,008,000
Activity in employee compensation plans	25,385,000	261,000	25,124,000	0	0	0
Stockholders' equity, ending balance at Jun. 30, 2017	1,244,463,000	10,277,000	527,624,000	20,000	706,542,000	0
Cumulative effect adjustment for adoption of ASUs (Notes 1 and 2)	(1,261,000)	0	0	13,000	(1,274,000)	0
Stockholders' equity, beginning balance at Dec. 31, 2017	1,345,560,000	10,280,000	535,815,000	63,000	799,402,000	0
Net income	16,015,000	0	0	0	13,653,000	2,362,000
Other comprehensive income (loss)	(141,000)	0	0	(141,000)	0	0
Total comprehensive income	15,874,000
Contributions	300,000,000	0	102,958,000	0	0	197,042,000
Transaction costs associated with sale of non-controlling interest	(2,254,000)	0	(2,254,000)	0	0	0
Tax effect of sale on non-controlling interest	(24,300,000)	0	(24,300,000)	0	0	0
Activity in employee compensation plans	10,035,000	134,000	9,901,000	0	0	0
Stockholders' equity, ending balance at Jun. 30, 2018	$ 1,643,654,000	$ 10,414,000	$ 622,120,000	$ (65,000)	$ 811,781,000	$ 199,404,000