Equity Level 4 (Details) - USD ($) $ / shares in Units, $ in Thousands	2 Months Ended	12 Months Ended
	Feb. 13, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Treasury Stock, Shares, Acquired	429,859	6,210,606	8,811,847
Common stock, par or stated value per share		$ 1	$ 1
Preferred Stock, Par or Stated Value Per Share		$ 1
Accumulated other comprehensive income [Roll Forward]
Balance at beginning of period	$ 41,943	$ 16,492
Other comprehensive income attributable to parent		25,462	$ 10,331	$ (53,283)
Balance at end of period		41,943	16,492
Treasury Stock, Value, Acquired, Par Value Method	$ 16,700	$ 199,600	$ 227,900
Treasury Stock Acquired, Average Cost Per Share	$ 38.83	$ 32.14	$ 25.87
Accumulated Foreign Currency Adjustment Attributable to Parent [Member]
Accumulated other comprehensive income [Roll Forward]
Balance at beginning of period	$ 41,943	$ 16,492	$ 6,164	59,414
Other comprehensive income attributable to parent		25,451	10,328	(53,250)
Decrease in net unrealized gains associated with available-for-sale securities of the trusts, net of taxes		0		0
Reclassification of net unrealized loss activity		0	0	0
Balance at end of period		41,943	16,492	6,164
Accumulated Net Unrealized Investment Gain (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Balance at beginning of period	0	0	0	0
Other comprehensive income attributable to parent		0	0	0
Decrease in net unrealized gains associated with available-for-sale securities of the trusts, net of taxes		243,677,000	120,573,288	(85,140)
Reclassification of net unrealized loss activity		(243,677,000)	(120,573,288)	85,140
Balance at end of period		0	0	0
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated other comprehensive income [Roll Forward]
Balance at beginning of period	$ 41,943	16,492	6,164	59,414
Other comprehensive income attributable to parent		25,451	10,328	(53,250)
Decrease in net unrealized gains associated with available-for-sale securities of the trusts, net of taxes		243,677,000	120,573,288	(85,140)
Reclassification of net unrealized loss activity		(243,677,000)	(120,573,288)	85,140
Balance at end of period		$ 41,943	$ 16,492	$ 6,164