Investments (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014 securities	Dec. 31, 2013 securities
Schedule of Investments [Line Items]
Available-for-sale securities in unrealized loss position, number of positions	644	1,364
Available-for-sale securities, estimated fair value	$ 2,451,210	$ 4,859,071
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(50,476)	(211,904)
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end	537	9
Available-for-sale securities, estimated fair value, 12 months or longer	1,919,820	31,247
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(43,551)	(546)
Fixed Maturities [Member]
Schedule of Investments [Line Items]
Available-for-sale securities, estimated fair value	2,095,620	4,810,249
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(48,921)	(210,786)
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end	537	8
Available-for-sale securities, estimated fair value, 12 months or longer	1,919,820	3,512
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	(43,551)	(146)
Fixed Maturities [Member] | Alterra Capital Holdings [Member]
Schedule of Investments [Line Items]
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end	523
Equity Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale securities, estimated fair value	40,595	48,822
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses	(1,554)	(1,118)
Number of available-for-sale securities positions in a continuous unrealized loss position for one year or longer at period end		1
Available-for-sale securities, estimated fair value, 12 months or longer	0	27,735
Available-for-sale securities, gross unrealized holding and other-than-temporary impairment losses, 12 months or longer	$ 0	$ (400)