Long-Term Debt (Tables)

12 Months Ended

Dec. 31, 2017

Long-Term Debt [Abstract]

Schedule of Long-Term Debt Instruments

	2017				2016
	Carrying Cost		Fair Value		Carrying Cost		Fair Value
6.35% Notes Due 2040	$	500,000		601,800		500,000		584,850
3.50% Notes Due 2027		500,000		488,300		—		—
6.30% Notes Due 2017		—		—		350,000		361,900
5.10% Notes Due 2044		300,000		313,320		300,000		297,600
3.15% Notes Due 2021		300,000		302,640		300,000		300,450
6.60% Debentures Due 2028		109,895		131,390		109,895		123,984
Total long-term debt		1,709,895		1,837,450		1,559,895		1,668,784
Less: Current portion		—		—		350,000		361,900
Less: Deferred debt expenses		16,286		—		11,216		—
Long-term debt	$	1,693,609		1,837,450		1,198,679		1,306,884