Financial Instruments (Available-for-sale Securities) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 410,500		$ 410,500		$ 576,021
Gross Unrealized Gains	51,015		51,015		525
Gross Unrealized Losses	(157)		(157)		(8,321)
Fair Value	461,358		461,358		568,225
Net unrealized gains (losses) on available-for-sale securities	32,585		32,585		(4,995)
Proceeds from sales and maturities of securities	275,573	$ 15,037	302,432	$ 15,037
Gross realized gains	1,280	3,795	1,341	3,795
Gross realized losses	(13)	$ 0	(22)	$ 0
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	60,281		60,281		54,371
Gross Unrealized Gains	49,194		49,194		466
Gross Unrealized Losses	(138)		(138)		(6,756)
Fair Value	109,337		109,337		48,081
Corporate bond securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	219,971		219,971		336,665
Gross Unrealized Gains	1,162		1,162		55
Gross Unrealized Losses	(16)		(16)		(1,076)
Fair Value	221,117		221,117		335,644
US government debt [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	72,702		72,702		135,226
Gross Unrealized Gains	462		462		2
Gross Unrealized Losses	0		0		(374)
Fair Value	73,164		73,164		134,854
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Cost	57,546		57,546		49,759
Gross Unrealized Gains	197		197		2
Gross Unrealized Losses	(3)		(3)		(115)
Fair Value	57,740		57,740		49,646
Available-for-sale Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Net of income tax expense (benefit) on available for sales securities	$ 18,273		$ 18,273		$ (2,801)