Consolidated Statement Of Equity - USD ($)	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Cumulative Dividends In Excess Of Net Income [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets				$ (235,000)	$ 235,000
Balance, shares at Dec. 31, 2017	41,532,154	41,532,154
Balance, value at Dec. 31, 2017	$ 1,322,117,000	$ 415,000	$ 1,289,919,000	32,605,000	(822,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss) attributable to common stockholders	117,251,000			117,251,000	3,100,000
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	120,351,000
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments	(2,427,000)		(2,427,000)
Stock Issued During Period, Shares, New Issues		668,072
Stock Issued During Period, Value, New Issues	47,893,000	$ 7,000	47,886,000
Adjustments Related to Tax Withholding for Share-based Compensation	(730,000)		(730,000)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		30,625
Share-based compensation	2,131,000		2,131,000
Dividends declared, $1.54 per share	(125,935,000)			(125,935,000)
Balance, value at Sep. 30, 2018	$ 1,363,400,000	$ 422,000	$ 1,336,779,000	$ 23,686,000	$ 2,513,000
Balance, shares at Sep. 30, 2018	42,230,851	42,230,851
Increase (Decrease) in Stockholders' Equity [Roll Forward]
New Accounting Pronouncement or Change in Accounting Principle, Cumulative Effect of Change on Equity or Net Assets	$ 235,000