Investments - Schedule of Investments Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended
Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Schedule of Investments [Line Items]
Fair value of securities transferred from AFS to HTM	$ 608,722
AFS Transferred to HTM securities, unrealized gain	$ 244
Available-for-sale fixed maturities, Original or amortized cost		$ 3,562,864	$ 3,379,864
Available-for-sale fixed maturities, Gross unrealized gains		58,115	116,051
Available-for-sale fixed maturities, Gross unrealized losses		(112,891)	(39,011)
Fair value		3,508,088	3,456,904
Fixed maturities, held-to-maturity, at amortized cost (fair value 2015: $598,975; 2014: $0)		607,843	0
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		3,458
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		(12,326)
HTM securities, Fair Value		598,975	0
Other Investments, Original or amortized cost		10,816	10,862
Other investments, Gross unrealized gains		1,091	1,709
Other investments, Gross unrealized losses		(95)	0
Other investments		11,812	12,571
Total investments, Original or amortized cost		4,181,523	3,390,726
Investments, Gross unrealized gains		62,664	117,760
Investments, Gross unrealized losses		(125,312)	(39,011)
Total investments, Fair value		4,118,875	3,469,475
U.S. treasury bonds
Schedule of Investments [Line Items]
Available-for-sale fixed maturities, Original or amortized cost		5,714	8,937
Available-for-sale fixed maturities, Gross unrealized gains		312	423
Available-for-sale fixed maturities, Gross unrealized losses		(16)	0
Fair value		6,010	9,360
U.S. agency bonds – mortgage-backed
Schedule of Investments [Line Items]
Available-for-sale fixed maturities, Original or amortized cost		1,471,782	1,313,834
Available-for-sale fixed maturities, Gross unrealized gains		15,399	19,197
Available-for-sale fixed maturities, Gross unrealized losses		(10,190)	(10,588)
Fair value		1,476,991	1,322,443
U.S. agency bonds – other
Schedule of Investments [Line Items]
Available-for-sale fixed maturities, Original or amortized cost		23,734	7,213
Available-for-sale fixed maturities, Gross unrealized gains		577	775
Available-for-sale fixed maturities, Gross unrealized losses		0	0
Fair value		24,311	7,988
Non-U.S. government and supranational bonds
Schedule of Investments [Line Items]
Available-for-sale fixed maturities, Original or amortized cost		35,128	54,467
Available-for-sale fixed maturities, Gross unrealized gains		0	304
Available-for-sale fixed maturities, Gross unrealized losses		(4,584)	(3,128)
Fair value		30,544	51,643
Asset-backed securities
Schedule of Investments [Line Items]
Available-for-sale fixed maturities, Original or amortized cost		165,719	52,337
Available-for-sale fixed maturities, Gross unrealized gains		1,174	2,443
Available-for-sale fixed maturities, Gross unrealized losses		(1,089)	0
Fair value		165,804	54,780
Corporate bonds
Schedule of Investments [Line Items]
Available-for-sale fixed maturities, Original or amortized cost		1,798,610	1,831,431
Available-for-sale fixed maturities, Gross unrealized gains		38,070	89,243
Available-for-sale fixed maturities, Gross unrealized losses		(97,012)	(25,295)
Fair value		1,739,668	1,895,379
Fixed maturities, held-to-maturity, at amortized cost (fair value 2015: $598,975; 2014: $0)		607,843
Held-to-maturity Securities, Accumulated Unrecognized Holding Gain		3,458
Held-to-maturity Securities, Accumulated Unrecognized Holding Loss		(12,326)
HTM securities, Fair Value		598,975
Municipal bonds
Schedule of Investments [Line Items]
Available-for-sale fixed maturities, Original or amortized cost		62,177	62,153
Available-for-sale fixed maturities, Gross unrealized gains		2,583	3,666
Available-for-sale fixed maturities, Gross unrealized losses		0	0
Fair value		$ 64,760	65,819
Short-term investments
Schedule of Investments [Line Items]
Available-for-sale fixed maturities, Original or amortized cost			49,492
Available-for-sale fixed maturities, Gross unrealized gains			0
Available-for-sale fixed maturities, Gross unrealized losses			0
Fair value			$ 49,492

Available-for-sale Securities, Transferred to Held To Maturity Securities, Unrealized Gain (Loss)

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Fair Value of Securities Transferred from Available for Sale Securities to Held to Maturity Securities

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Investments, Original or Amortized Cost Basis

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Fair value of investments in debt securities classified as held-to-maturity.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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