Debt and financing arrangements (Credit and other facilities) (Details) - USD ($) $ in Thousands		Dec. 31, 2017		Aug. 07, 2017	Dec. 31, 2016	Dec. 09, 2015
Line of Credit Facility [Line Items]
Credit facility, commitment		$ 1,319,096			$ 690,000
Letters of credit issued and outstanding		428,180			311,163
Drawn		206,000
Cash and investments pledged as collateral		576,864			442,184
$85,000 syndicated unsecured letter of credit facility
Line of Credit Facility [Line Items]
Credit facility, commitment		85,000			85,000	$ 85,000
Letters of credit issued and outstanding	[1]	0			0
Cash and investments pledged as collateral					0
$300,000 syndicated secured letter of credit facility
Line of Credit Facility [Line Items]
Credit facility, commitment		300,000			300,000	$ 300,000
Letters of credit issued and outstanding	[1]	92,979			121,428
Cash and investments pledged as collateral		118,188			157,597
$24,000 secured bi-lateral letter of credit facility
Line of Credit Facility [Line Items]
Credit facility, commitment		24,000			24,000
Letters of credit issued and outstanding	[1]	5,765			4,553
Cash and investments pledged as collateral		22,340			48,097
AlphaCat Re secured letter of credit facility
Line of Credit Facility [Line Items]
Credit facility, commitment	[2]				20,000
Letters of credit issued and outstanding	[1],[2]				20,000
Cash and investments pledged as collateral	[2]				20,032
$25,000 IPC bi-lateral facility
Line of Credit Facility [Line Items]
Credit facility, commitment		25,000			25,000
Letters of credit issued and outstanding	[1]	7,754			8,807
Cash and investments pledged as collateral					0
$236,000 Flagstone bi-lateral facility
Line of Credit Facility [Line Items]
Credit facility, commitment		236,000			236,000
Letters of credit issued and outstanding	[1]	115,682			156,375
Cash and investments pledged as collateral		184,569			216,458
$65,000 unsecured revolving credit facility
Line of Credit Facility [Line Items]
Credit facility, commitment		65,000		$ 65,000
Letters of credit issued and outstanding		0	[1]		$ 0
Drawn	[3]	0
$100,000 unsecured revolving credit facility
Line of Credit Facility [Line Items]
Credit facility, commitment		100,000		$ 100,000
Letters of credit issued and outstanding	[1]	0
Drawn	[3]	0
FHLB secured facility
Line of Credit Facility [Line Items]
Credit facility, commitment		484,096
Letters of credit issued and outstanding	[1]	206,000
Drawn	[3]	206,000
Cash and investments pledged as collateral		$ 251,767

[1] Indicates utilization of commitment amount.

[2] The Company terminated its AlphaCat Re secured letter of credit facility on January 6, 2017.

[3] Represents drawn borrowings included in accounts payable and accrued expenses.