Condensed Consolidated Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands		Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Loss	Total stockholders' equity	Noncontrolling Interests
Balance value at Dec. 31, 2016		$ 3,014,349	$ 1	$ 2,191,756	$ 1,046,822	$ (230,400)	$ 3,008,179	$ 6,170
Balance shares at Dec. 31, 2016			109,944
Net income (loss)		58,069			58,282		58,282	(213)
Dividends on common shares		(37,495)			(37,495)		(37,495)
Shares issued in connection with share-based compensation, value		(5,947)		(5,078)	(869)		(5,947)
Shares issued in connection with share-based compensation, shares			584
Share-based compensation		17,032		17,032			17,032
Net activity related to noncontrolling interests		(841)						(841)
Other comprehensive income (loss)		32,418				32,411	32,411	7
Balance value at Mar. 31, 2017		3,077,585	$ 1	2,203,710	1,066,740	(197,989)	3,072,462	5,123
Balance shares at Mar. 31, 2017			110,528
Balance value at Dec. 31, 2016		3,014,349	$ 1	2,191,756	1,046,822	(230,400)	3,008,179	6,170
Balance shares at Dec. 31, 2016			109,944
Net income (loss)		201,150
Other comprehensive income (loss)		168,633
Balance value at Sep. 30, 2017		3,298,397	$ 1	2,226,691	1,128,981	(61,779)	3,293,894	4,503
Balance shares at Sep. 30, 2017			110,817
Balance value at Mar. 31, 2017		3,077,585	$ 1	2,203,710	1,066,740	(197,989)	3,072,462	5,123
Balance shares at Mar. 31, 2017			110,528
Net income (loss)		121,895			122,257		122,257	(362)
Dividends on common shares		(37,604)			(37,604)		(37,604)
Shares issued in connection with share-based compensation, value		3,780		4,591	(811)		3,780
Shares issued in connection with share-based compensation, shares			193
Share-based compensation		7,548		7,548			7,548
Net activity related to noncontrolling interests		(26)						(26)
Other comprehensive income (loss)		24,980				24,980	24,980
Balance value at Jun. 30, 2017		3,198,158	$ 1	2,215,849	1,150,582	(173,009)	3,193,423	4,735
Balance shares at Jun. 30, 2017			110,721
Net income (loss)		21,186			21,383		21,383	(197)
Dividends on common shares		(42,075)			(42,075)		(42,075)
Shares issued in connection with share-based compensation, value		3,362		4,271	(909)		3,362
Shares issued in connection with share-based compensation, shares			96
Share-based compensation		6,616		6,616			6,616
Net activity related to noncontrolling interests		(85)		(45)			(45)	(40)
Other comprehensive income (loss)		111,235				111,230	111,230	5
Balance value at Sep. 30, 2017		3,298,397	$ 1	2,226,691	1,128,981	(61,779)	3,293,894	4,503
Balance shares at Sep. 30, 2017			110,817
Balance value at Dec. 31, 2017		$ 3,483,025	$ 1	2,236,351	1,311,112	(67,509)	3,479,955	3,070
Balance shares at Dec. 31, 2017		110,925	110,925
Cumulative-effect adjustment (Note 1) | Accounting Standards Update 2016-01					40,550	(40,550)
Net income (loss)		$ 76,172			76,227		76,227	(55)
Dividends on common shares		(42,330)			(42,330)		(42,330)
Shares issued in connection with share-based compensation, value		(12,606)		(11,759)	(847)		(12,606)
Shares issued in connection with share-based compensation, shares			620
Share-based compensation		19,509		19,509			19,509
Net activity related to noncontrolling interests		(901)		189			189	(1,090)
Other comprehensive income (loss)		(50,428)				(50,409)	(50,409)	(19)
Balance value at Mar. 31, 2018		3,472,441	$ 1	2,244,290	1,384,712	(158,468)	3,470,535	1,906
Balance shares at Mar. 31, 2018			111,545
Balance value at Dec. 31, 2017		$ 3,483,025	$ 1	2,236,351	1,311,112	(67,509)	3,479,955	3,070
Balance shares at Dec. 31, 2017		110,925	110,925
Cumulative-effect adjustment (Note 1) | Accounting Standards Update 2016-01	[1]					(40,550)
Net income (loss)		$ 382,724
Other comprehensive income (loss)		(84,872)
Balance value at Sep. 30, 2018		$ 3,675,998	$ 1	2,266,830	1,600,296	(192,912)	3,674,215	1,783
Balance shares at Sep. 30, 2018		111,787	111,787
Balance value at Mar. 31, 2018		$ 3,472,441	$ 1	2,244,290	1,384,712	(158,468)	3,470,535	1,906
Balance shares at Mar. 31, 2018			111,545
Net income (loss)		155,091			155,140		155,140	(49)
Dividends on common shares		(42,387)			(42,387)		(42,387)
Shares issued in connection with share-based compensation, value		1,377		2,204	(827)		1,377
Shares issued in connection with share-based compensation, shares			113
Share-based compensation		7,626		7,626			7,626
Net activity related to noncontrolling interests		(25)		133			133	(158)
Other comprehensive income (loss)		(22,901)				(22,901)	(22,901)
Balance value at Jun. 30, 2018		3,571,222	$ 1	2,254,253	1,496,638	(181,369)	3,569,523	1,699
Balance shares at Jun. 30, 2018			111,658
Net income (loss)		151,461			151,480		151,480	(19)
Dividends on common shares		(46,912)			(46,912)		(46,912)
Shares issued in connection with share-based compensation, value		4,556		5,466	(910)		4,556
Shares issued in connection with share-based compensation, shares			129
Share-based compensation		7,241		7,241			7,241
Net activity related to noncontrolling interests		(27)		(130)			(130)	103
Other comprehensive income (loss)		(11,543)				(11,543)	(11,543)
Balance value at Sep. 30, 2018		$ 3,675,998	$ 1	$ 2,266,830	$ 1,600,296	$ (192,912)	$ 3,674,215	$ 1,783
Balance shares at Sep. 30, 2018		111,787	111,787

[1] The Company recognized a cumulative-effect adjustment to retained earnings for cumulative net unrealized gains related to its investments in equity securities upon adoption of new accounting guidance on January 1, 2018. See Note 1 Basis of Condensed Consolidated Financial Statements for further discussion of the new guidance.