Investments (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Investment [Line Items]
Proceeds from available-for-sale of investments	$ 401,700,000	$ 606,500,000	$ 834,600,000	$ 872,900,000
Average remaining maturities for short-term available for sale investments			6 months
Average remaining maturities for long-term available for sale investments			3 years
Available-for-sale Securities, Gross Unrealized Loss Accumulated in Investments	1,225,000		$ 1,225,000		$ 2,116,000
Cost method investments, gain recorded on cost method investments	0		0
Other Assets [Member]
Investment [Line Items]
Cost method investments	15,000,000		15,000,000		$ 16,600,000
Other Income (Expense) [Member]
Investment [Line Items]
Realized gains on the sales of available-for-sale investments	300,000	900,000	400,000	1,200,000
Realized losses on the sales of available-for-sale investments	100,000	200,000	300,000	300,000
Level 3 [Member] | Fair Value, Measurements, Nonrecurring [Member] | Other Income (Expense) [Member]
Investment [Line Items]
Cost method investment, impairment charge included other income	$ 2,500,000	$ 3,200,000	$ 3,000,000	8,300,000
Cost method investments, gain recorded on cost method investments				$ 1,500,000