Debt (Tables)

12 Months Ended

Dec. 31, 2017

Debt Disclosure [Abstract]

Schedule of Long-term Debt Instruments [Table Text Block]

The Company’s long-term debt consists of the following as of December 31, 2017 and 2016 (dollars in thousands):

	2017			2016
$300 million 5.625% Senior Notes due 2017	$	—		$	299,945
$400 million 6.45% Senior Notes due 2019	399,873			399,805
$400 million 5.00% Senior Notes due 2021	399,245			399,025
$400 million 4.70% Senior Notes due 2023	399,138			398,986
$400 million 3.95% Senior Notes due 2026	399,987			399,985
$100 million 4.09% Promissory Note due 2039	91,787			94,370
$400 million 6.20% Subordinated Debentures due 2042	400,000			400,000
$400 million 5.75% Subordinated Debentures due 2056	400,000			400,000
$400 million Variable Rate Junior Subordinated Debentures due 2065	318,740			318,737
Sub-total	2,808,770			3,110,853
Unamortized issuance costs	(20,405		)	(22,218		)
Long-term Debt	$	2,788,365		$	3,088,635

Schedule of Maturities of Long-term Debt [Table Text Block]

Future principal payments due on long-term debt, excluding discounts, as of December 31, 2017, were as follows (dollars in thousands):

	Calendar Year
	2018		2019		2020		2021		2022		Thereafter
Long-term debt	$	2,690	$	402,802	$	2,919	$	403,040	$	3,167	$	1,996,972

Schedule of Line of Credit Facilities [Table Text Block]

The following table provides additional information on the Company’s existing committed credit facilities as of December 31, 2017 and 2016 (dollars in thousands):

		Amount Utilized(1) December 31,
Current Capacity	Maturity Date	2017		2016		Basis of Fees
$180,403 (2)	November 2018	$	8,638	$	31,382	Fixed
5,952 (2)	March 2019	5,952		17,513		Fixed
850,000	September 2019	96,564		96,095		Senior unsecured long-term debt rating
188,000	October 2019	188,000		270,000		Fixed
117,135 (2)	December 2019	117,135		72,080		Fixed
100,000	June 2020	75,573		70,690		Fixed
100,000	May 2021	100,000		—		Fixed
75,000	June 2021	61,900		85,040		Fixed
500,000	May 2022	500,000		—		Fixed

(1) Represents issued but undrawn letters of credit. There was no cash borrowed for the periods presented.

(2) Foreign currency denominated facility, amounts presented are in U.S. dollars.