Fair Value of Financial Instruments (Fair Value Measurements for Level 3 Instruments) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	$ 134,916	$ 0	$ 169,314	$ 0
Total realized and unrealized gains (losses):
Included in net income	45,251	0	45,799	0
Included in other comprehensive income	0	0	0	0
Purchases, sales, issuances and settlements, net	(33,653)	0	(68,599)	0
Transfers into (out of) Level 3	0	0	0	0
Balance at end of period	146,514	0	146,514	0
Amount of total gains (losses) for the period included in net income attributable to the change in unrealized gains (losses) relating to assets still held at the end of period		0	44,200	0
Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	160,536	4,073	193,338	2,718
Total realized and unrealized gains (losses):
Included in net income	44,779	1,484	47,430	2,966
Included in comprehensive income	0	0	0	0
Purchases, sales, issuances and settlements, net	(33,887)	(1,187)	69,340	1,314
Transfers into (out of) Level 3	0	0	0	0
Balance at end of period	171,428	4,370	171,428	4,370
Fair Value, Liabilities Measured on Recurring Basis, Change in Unrealized Gain (Loss)	1,006	2,190	3,091	3,742
Debt Securities, Available for Sale [Member]
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	0	0	0	0
Total realized and unrealized gains (losses):
Included in net income	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases, sales, issuances and settlements, net		0	0	0
Transfers into (out of) Level 3	0	0	0	0
Balance at end of period	0	0	0	0
Amount of total gains (losses) for the period included in net income attributable to the change in unrealized gains (losses) relating to assets still held at the end of period	0	0	0	0
Equity Securities, Available for Sale [Member]
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	0	0	0	0
Total realized and unrealized gains (losses):
Included in net income	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases, sales, issuances and settlements, net		0	0	0
Transfers into (out of) Level 3	0	0	0	0
Balance at end of period	0	0	0	0
Amount of total gains (losses) for the period included in net income attributable to the change in unrealized gains (losses) relating to assets still held at the end of period	0	0	0	0
Derivatives, Index Options [Member]
Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Beginning balance	134,916	0	169,314
Total realized and unrealized gains (losses):
Included in net income	45,251	0	45,799
Included in other comprehensive income		0	0
Purchases, sales, issuances and settlements, net	(33,653)	0	(68,599)
Transfers into (out of) Level 3		0	0
Balance at end of period	146,514	0	146,514	0
Amount of total gains (losses) for the period included in net income attributable to the change in unrealized gains (losses) relating to assets still held at the end of period	$ 43,313	$ 0	$ 44,200