Accumulated Other Comprehensive Income ("AOCI") (AOCI by component) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, value			$ 5,007,701	$ 7,618,496
Cumulative effect of accounting change (see Note 3)					$ 14,519
Ending balance, value	$ 5,037,509	$ 5,287,573	5,037,509	5,287,573
Interest rate swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, value	39,559	12,702	23,542	8,066
Cumulative effect of accounting change (see Note 3)					2,851
OCI before reclassifications	623	1,976	13,789	6,612
Amounts reclassified from AOCI	0	0	0	0
Net current period OCI	623	1,976	13,789	6,612
Ending balance, value	40,182	14,678	40,182	14,678
Accumulated Other Comprehensive Income
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, value	33,351	115,839	128,682	118,972
Cumulative effect of accounting change (see Note 3)					(108,374)
OCI before reclassifications	822	6,608	13,865	(5,793)
Amounts reclassified from AOCI	0	(646)	0	8,622
Net current period OCI	822	5,962	13,865	2,829
Ending balance, value	34,173	121,801	34,173	121,801
Marketable securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, value	0	114,290	109,554	130,505
Cumulative effect of accounting change (see Note 3)					(109,554)
OCI before reclassifications	0	5,656	0	(10,559)
Amounts reclassified from AOCI	0	0	0	0
Net current period OCI	0	5,656	0	(10,559)
Ending balance, value	0	119,946	0	119,946
Pro-rata share of nonconsolidated subsidiaries' OCI
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, value	2,834	(3,821)	3,769	(12,058)
Cumulative effect of accounting change (see Note 3)					(1,671)
OCI before reclassifications	253	(626)	989	(1,657)
Amounts reclassified from AOCI	0	(646)	0	8,622
Net current period OCI	253	(1,272)	989	6,965
Ending balance, value	3,087	(5,093)	3,087	(5,093)
Other
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance, value	(9,042)	(7,332)	(8,183)	(7,541)
Cumulative effect of accounting change (see Note 3)					$ 0
OCI before reclassifications	(54)	(398)	(913)	(189)
Amounts reclassified from AOCI	0	0	0	0
Net current period OCI	(54)	(398)	(913)	(189)
Ending balance, value	$ (9,096)	$ (7,730)	$ (9,096)	$ (7,730)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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