Summary of Significant Accounting Policies (New Accounting Pronouncements) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating, selling, administrative and general expenses	$ 87,549	$ 82,682	$ 262,961	$ 249,654
Operating income	66,018	59,723	175,963	187,444
Other, net	2,048	1,374	4,761	3,452
Income before provision for income taxes	29,714	26,950	67,082	70,316
Decrease in restricted assets			254	2,112
Net cash used in investing activities			27,362	(45,772)
Net increase in cash, cash equivalents and restricted cash			59,992	8,183
Cash, cash equivalents and restricted cash, beginning of period			310,937	398,578
Cash, cash equivalents and restricted cash, end of period	$ 370,929	406,761	$ 370,929	406,761
Impact of adoption of new accounting standards					$ 12,857	$ (8,838)
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of adoption of new accounting standards					(1,094)
AOCI Attributable to Parent
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of adoption of new accounting standards					6,036
Accounting Standards Update 2016-01
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of adoption of new accounting standards					8,838
Accounting Standards Update 2016-01 | Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of adoption of new accounting standards					14,874
Accounting Standards Update 2016-01 | AOCI Attributable to Parent
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Impact of adoption of new accounting standards					$ 6,036
As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating, selling, administrative and general expenses		83,172		251,124
Operating income		59,233		185,974
Other, net		1,864		4,922
Income before provision for income taxes		26,950		70,316
Decrease in restricted assets				(3,541)
Net cash used in investing activities				(51,425)
Net increase in cash, cash equivalents and restricted cash				2,530
Cash, cash equivalents and restricted cash, beginning of period				393,530
Cash, cash equivalents and restricted cash, end of period		396,060		396,060
Restatement Adjustment | Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating, selling, administrative and general expenses		(490)		(1,470)
Operating income		490		1,470
Other, net		(490)		(1,470)
Income before provision for income taxes		0		0
Restatement Adjustment | Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in restricted assets				5,653
Net cash used in investing activities				5,653
Net increase in cash, cash equivalents and restricted cash				5,653
Cash, cash equivalents and restricted cash, beginning of period				5,048
Cash, cash equivalents and restricted cash, end of period		$ 10,701		$ 10,701