Derivatives (Narrative) (Details)	6 Months Ended
	Jun. 30, 2015 USD ($) item security	Dec. 31, 2005 security	Dec. 31, 2001 security
Derivative [Line Items]
Number of equity index put option contracts sold | security		7	7
Number of equity indices | item	2
Standard & Poor's 500 Index [Member]
Derivative [Line Items]
Number of equity index put option contracts sold | security	6
Theoretical maximum payout occurs based on index value	$ 0
Discount factor	3.00%
Present value of theoretical maximum payouts	$ 426,214,000
Equity index if contracts had expired | item	2,063.11
Settlement amount if contracts had expired	$ 0
Standard & Poor's 500 Index [Member] | Equity Index Put Option Contracts [Member]
Derivative [Line Items]
Total consideration, net of commissions	22,530,000
Standard & Poor's 500 Index [Member] | Equity Index Put Option Contracts [Member] | Fair Value [Member]
Derivative [Line Items]
Total consideration, net of commissions	$ 32,173,000
Standard & Poor's 500 Index [Member] | Maximum [Member]
Derivative [Line Items]
Estimates the probability of equity index put option contract will falling below the strike price on the exercise date	18.00%
FTSE 100 Index [Member]
Derivative [Line Items]
Number of equity index put option contracts sold | security	1
Theoretical maximum payout occurs based on index value	$ 0
Discount factor	3.00%
Present value of theoretical maximum payouts	$ 43,847,000
Equity index if contracts had expired | item	6,521.00
Settlement amount if contracts had expired	$ 0
FTSE 100 Index [Member] | Equity Index Put Option Contracts [Member]
Derivative [Line Items]
Total consideration, net of commissions	6,706,000
FTSE 100 Index [Member] | Equity Index Put Option Contracts [Member] | Fair Value [Member]
Derivative [Line Items]
Total consideration, net of commissions	$ 8,646,000
FTSE 100 Index [Member] | Maximum [Member]
Derivative [Line Items]
Estimates the probability of equity index put option contract will falling below the strike price on the exercise date	42.00%