Senior Notes (Summary of Senior Notes) (Details) - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 31, 2014
Debt Instrument [Line Items]
Long-term Debt, gross		$ 800,000	$ 800,000
Debt issuance costs, net		(3,704)	(4,109)
Long-term Debt		796,296	795,891
Senior notes 4.250% due 2024 [Member]
Debt Instrument [Line Items]
Long-term Debt, gross	[1]	500,000	500,000
Long-term Debt					$ 300,000
Senior notes 3.50% due 2020 [Member]
Debt Instrument [Line Items]
Long-term Debt, gross	[2]	$ 300,000	$ 300,000
Long-term Debt				$ 300,000

[1] In July 2014, we sold in a registered underwritten public offering, $300.0 million in aggregate principal amount of 4.250% senior notes due July 2024. Interest on these senior notes is payable semi-annually in arrears. We may redeem the notes in whole or in part, at our option, at a redemption price equal to 100% of their principal amount, plus a “make-whole” premium and accrued and unpaid interest, if any, to the date of redemption. In July 2016, we issued an additional $200.0 million in aggregate principal amount of 4.25% senior notes due 2024.

[2] In December 2015, we sold in a registered underwritten public offering, $300.0 million in aggregate principal amount of 3.50% senior notes due December 2020. Interest on these senior notes is payable semi-annually in arrears. We may redeem the notes in whole or in part, at our option, at a redemption price equal to 100% of their principal amount, plus a “make-whole” premium and accrued and unpaid interest, if any, to the date of redemption.